diff --git a/index.d.ts b/index.d.ts
index 26de25c576..38a05c1249 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -21,8 +21,7 @@ type AxiosHeaderTester = (matcher?: AxiosHeaderMatcher) => boolean;
 
 export class AxiosHeaders {
   constructor(
-      headers?: RawAxiosHeaders | AxiosHeaders,
-      defaultHeaders?: RawAxiosHeaders | AxiosHeaders
+      headers?: RawAxiosHeaders | AxiosHeaders
   );
 
   set(headerName?: string, value?: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
@@ -39,12 +38,16 @@ export class AxiosHeaders {
 
   normalize(format: boolean): AxiosHeaders;
 
+  concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+
   toJSON(asStrings?: boolean): RawAxiosHeaders;
 
   static from(thing?: AxiosHeaders | RawAxiosHeaders | string): AxiosHeaders;
 
   static accessor(header: string | string[]): AxiosHeaders;
 
+  static concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+
   setContentType: AxiosHeaderSetter;
   getContentType: AxiosHeaderGetter;
   hasContentType: AxiosHeaderTester;
diff --git a/lib/adapters/http.js b/lib/adapters/http.js
index 9e56fdad6b..b561645d2f 100755
--- a/lib/adapters/http.js
+++ b/lib/adapters/http.js
@@ -203,7 +203,7 @@ export default function httpAdapter(config) {
         data: convertedData,
         status: 200,
         statusText: 'OK',
-        headers: {},
+        headers: new AxiosHeaders(),
         config
       });
     }
@@ -588,4 +588,4 @@ export default function httpAdapter(config) {
   });
 }
 
-export const __setProxy = setProxy;
\ No newline at end of file
+export const __setProxy = setProxy;
diff --git a/lib/axios.js b/lib/axios.js
index 312ec95c17..ed9d662444 100644
--- a/lib/axios.js
+++ b/lib/axios.js
@@ -75,5 +75,6 @@ axios.AxiosHeaders = AxiosHeaders;
 axios.formToJSON = thing => formDataToJSON(utils.isHTMLForm(thing) ? new FormData(thing) : thing);
 
 axios.default = axios;
+
 // this module should only have a default export
 export default axios
diff --git a/lib/core/Axios.js b/lib/core/Axios.js
index 31f8b531dc..ff602ba52c 100644
--- a/lib/core/Axios.js
+++ b/lib/core/Axios.js
@@ -47,7 +47,7 @@ class Axios {
 
     config = mergeConfig(this.defaults, config);
 
-    const {transitional, paramsSerializer} = config;
+    const {transitional, paramsSerializer, headers} = config;
 
     if (transitional !== undefined) {
       validator.assertOptions(transitional, {
@@ -67,20 +67,22 @@ class Axios {
     // Set config.method
     config.method = (config.method || this.defaults.method || 'get').toLowerCase();
 
+    let contextHeaders;
+
     // Flatten headers
-    const defaultHeaders = config.headers && utils.merge(
-      config.headers.common,
-      config.headers[config.method]
+    contextHeaders = headers && utils.merge(
+      headers.common,
+      headers[config.method]
     );
 
-    defaultHeaders && utils.forEach(
+    contextHeaders && utils.forEach(
       ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
-      function cleanHeaderConfig(method) {
-        delete config.headers[method];
+      (method) => {
+        delete headers[method];
       }
     );
 
-    config.headers = new AxiosHeaders(config.headers, defaultHeaders);
+    config.headers = AxiosHeaders.concat(contextHeaders, headers);
 
     // filter out skipped interceptors
     const requestInterceptorChain = [];
diff --git a/lib/core/AxiosHeaders.js b/lib/core/AxiosHeaders.js
index 68e098a0f7..b2f1c2fe76 100644
--- a/lib/core/AxiosHeaders.js
+++ b/lib/core/AxiosHeaders.js
@@ -4,7 +4,6 @@ import utils from '../utils.js';
 import parseHeaders from '../helpers/parseHeaders.js';
 
 const $internals = Symbol('internals');
-const $defaults = Symbol('defaults');
 
 function normalizeHeader(header) {
   return header && String(header).trim().toLowerCase();
@@ -30,6 +29,10 @@ function parseTokens(str) {
   return tokens;
 }
 
+function isValidHeaderName(str) {
+  return /^[-_a-zA-Z]+$/.test(str.trim());
+}
+
 function matchHeaderValue(context, value, header, filter) {
   if (utils.isFunction(filter)) {
     return filter.call(this, value, header);
@@ -66,27 +69,12 @@ function buildAccessors(obj, header) {
   });
 }
 
-function findKey(obj, key) {
-  key = key.toLowerCase();
-  const keys = Object.keys(obj);
-  let i = keys.length;
-  let _key;
-  while (i-- > 0) {
-    _key = keys[i];
-    if (key === _key.toLowerCase()) {
-      return _key;
-    }
+class AxiosHeaders {
+  constructor(headers) {
+    headers && this.set(headers);
   }
-  return null;
-}
-
-function AxiosHeaders(headers, defaults) {
-  headers && this.set(headers);
-  this[$defaults] = defaults || null;
-}
 
-Object.assign(AxiosHeaders.prototype, {
-  set: function(header, valueOrRewrite, rewrite) {
+  set(header, valueOrRewrite, rewrite) {
     const self = this;
 
     function setHeader(_value, _header, _rewrite) {
@@ -96,69 +84,70 @@ Object.assign(AxiosHeaders.prototype, {
         throw new Error('header name must be a non-empty string');
       }
 
-      const key = findKey(self, lHeader);
+      const key = utils.findKey(self, lHeader);
 
-      if (key && _rewrite !== true && (self[key] === false || _rewrite === false)) {
-        return;
+      if(!key || self[key] === undefined || _rewrite === true || (_rewrite === undefined && self[key] !== false)) {
+        self[key || _header] = normalizeValue(_value);
       }
-
-      self[key || _header] = normalizeValue(_value);
     }
 
-    if (utils.isPlainObject(header)) {
-      utils.forEach(header, (_value, _header) => {
-        setHeader(_value, _header, valueOrRewrite);
-      });
+    const setHeaders = (headers, _rewrite) =>
+      utils.forEach(headers, (_value, _header) => setHeader(_value, _header, _rewrite));
+
+    if (utils.isPlainObject(header) || header instanceof this.constructor) {
+      setHeaders(header, valueOrRewrite)
+    } else if(utils.isString(header) && (header = header.trim()) && !isValidHeaderName(header)) {
+      setHeaders(parseHeaders(header), valueOrRewrite);
     } else {
-      setHeader(valueOrRewrite, header, rewrite);
+      header != null && setHeader(valueOrRewrite, header, rewrite);
     }
 
     return this;
-  },
+  }
 
-  get: function(header, parser) {
+  get(header, parser) {
     header = normalizeHeader(header);
 
-    if (!header) return undefined;
+    if (header) {
+      const key = utils.findKey(this, header);
 
-    const key = findKey(this, header);
+      if (key) {
+        const value = this[key];
 
-    if (key) {
-      const value = this[key];
+        if (!parser) {
+          return value;
+        }
 
-      if (!parser) {
-        return value;
-      }
+        if (parser === true) {
+          return parseTokens(value);
+        }
 
-      if (parser === true) {
-        return parseTokens(value);
-      }
+        if (utils.isFunction(parser)) {
+          return parser.call(this, value, key);
+        }
 
-      if (utils.isFunction(parser)) {
-        return parser.call(this, value, key);
-      }
+        if (utils.isRegExp(parser)) {
+          return parser.exec(value);
+        }
 
-      if (utils.isRegExp(parser)) {
-        return parser.exec(value);
+        throw new TypeError('parser must be boolean|regexp|function');
       }
-
-      throw new TypeError('parser must be boolean|regexp|function');
     }
-  },
+  }
 
-  has: function(header, matcher) {
+  has(header, matcher) {
     header = normalizeHeader(header);
 
     if (header) {
-      const key = findKey(this, header);
+      const key = utils.findKey(this, header);
 
       return !!(key && (!matcher || matchHeaderValue(this, this[key], key, matcher)));
     }
 
     return false;
-  },
+  }
 
-  delete: function(header, matcher) {
+  delete(header, matcher) {
     const self = this;
     let deleted = false;
 
@@ -166,7 +155,7 @@ Object.assign(AxiosHeaders.prototype, {
       _header = normalizeHeader(_header);
 
       if (_header) {
-        const key = findKey(self, _header);
+        const key = utils.findKey(self, _header);
 
         if (key && (!matcher || matchHeaderValue(self, self[key], key, matcher))) {
           delete self[key];
@@ -183,18 +172,18 @@ Object.assign(AxiosHeaders.prototype, {
     }
 
     return deleted;
-  },
+  }
 
-  clear: function() {
+  clear() {
     return Object.keys(this).forEach(this.delete.bind(this));
-  },
+  }
 
-  normalize: function(format) {
+  normalize(format) {
     const self = this;
     const headers = {};
 
     utils.forEach(this, (value, header) => {
-      const key = findKey(headers, header);
+      const key = utils.findKey(headers, header);
 
       if (key) {
         self[key] = normalizeValue(value);
@@ -214,30 +203,47 @@ Object.assign(AxiosHeaders.prototype, {
     });
 
     return this;
-  },
+  }
+
+  concat(...targets) {
+    return this.constructor.concat(this, ...targets);
+  }
 
-  toJSON: function(asStrings) {
+  toJSON(asStrings) {
     const obj = Object.create(null);
 
-    utils.forEach(Object.assign({}, this[$defaults] || null, this),
-      (value, header) => {
-        if (value == null || value === false) return;
-        obj[header] = asStrings && utils.isArray(value) ? value.join(', ') : value;
-      });
+    utils.forEach(this, (value, header) => {
+      value != null && value !== false && (obj[header] = asStrings && utils.isArray(value) ? value.join(', ') : value);
+    });
 
     return obj;
   }
-});
 
-Object.assign(AxiosHeaders, {
-  from: function(thing) {
-    if (utils.isString(thing)) {
-      return new this(parseHeaders(thing));
-    }
+  [Symbol.iterator]() {
+    return Object.entries(this.toJSON())[Symbol.iterator]();
+  }
+
+  toString() {
+    return Object.entries(this.toJSON()).map(([header, value]) => header + ': ' + value).join('\n');
+  }
+
+  get [Symbol.toStringTag]() {
+    return 'AxiosHeaders';
+  }
+
+  static from(thing) {
     return thing instanceof this ? thing : new this(thing);
-  },
+  }
 
-  accessor: function(header) {
+  static concat(first, ...targets) {
+    const computed = new this(first);
+
+    targets.forEach((target) => computed.set(target));
+
+    return computed;
+  }
+
+  static accessor(header) {
     const internals = this[$internals] = (this[$internals] = {
       accessors: {}
     });
@@ -258,7 +264,7 @@ Object.assign(AxiosHeaders, {
 
     return this;
   }
-});
+}
 
 AxiosHeaders.accessor(['Content-Type', 'Content-Length', 'Accept', 'Accept-Encoding', 'User-Agent']);
 
diff --git a/lib/core/dispatchRequest.js b/lib/core/dispatchRequest.js
index 46ced28e33..dfe4c41d66 100644
--- a/lib/core/dispatchRequest.js
+++ b/lib/core/dispatchRequest.js
@@ -41,6 +41,10 @@ export default function dispatchRequest(config) {
     config.transformRequest
   );
 
+  if (['post', 'put', 'patch'].indexOf(config.method) !== -1) {
+    config.headers.setContentType('application/x-www-form-urlencoded', false);
+  }
+
   const adapter = config.adapter || defaults.adapter;
 
   return adapter(config).then(function onAdapterResolution(response) {
diff --git a/lib/core/mergeConfig.js b/lib/core/mergeConfig.js
index 328e631824..2aee6b895c 100644
--- a/lib/core/mergeConfig.js
+++ b/lib/core/mergeConfig.js
@@ -1,6 +1,9 @@
 'use strict';
 
 import utils from '../utils.js';
+import AxiosHeaders from "./AxiosHeaders.js";
+
+const headersToObject = (thing) => thing instanceof AxiosHeaders ? thing.toJSON() : thing;
 
 /**
  * Config-specific merge-function which creates a new config-object
@@ -16,9 +19,9 @@ export default function mergeConfig(config1, config2) {
   config2 = config2 || {};
   const config = {};
 
-  function getMergedValue(target, source) {
+  function getMergedValue(target, source, caseless) {
     if (utils.isPlainObject(target) && utils.isPlainObject(source)) {
-      return utils.merge(target, source);
+      return utils.merge.call({caseless}, target, source);
     } else if (utils.isPlainObject(source)) {
       return utils.merge({}, source);
     } else if (utils.isArray(source)) {
@@ -28,72 +31,73 @@ export default function mergeConfig(config1, config2) {
   }
 
   // eslint-disable-next-line consistent-return
-  function mergeDeepProperties(prop) {
-    if (!utils.isUndefined(config2[prop])) {
-      return getMergedValue(config1[prop], config2[prop]);
-    } else if (!utils.isUndefined(config1[prop])) {
-      return getMergedValue(undefined, config1[prop]);
+  function mergeDeepProperties(a, b, caseless) {
+    if (!utils.isUndefined(b)) {
+      return getMergedValue(a, b, caseless);
+    } else if (!utils.isUndefined(a)) {
+      return getMergedValue(undefined, a, caseless);
     }
   }
 
   // eslint-disable-next-line consistent-return
-  function valueFromConfig2(prop) {
-    if (!utils.isUndefined(config2[prop])) {
-      return getMergedValue(undefined, config2[prop]);
+  function valueFromConfig2(a, b) {
+    if (!utils.isUndefined(b)) {
+      return getMergedValue(undefined, b);
     }
   }
 
   // eslint-disable-next-line consistent-return
-  function defaultToConfig2(prop) {
-    if (!utils.isUndefined(config2[prop])) {
-      return getMergedValue(undefined, config2[prop]);
-    } else if (!utils.isUndefined(config1[prop])) {
-      return getMergedValue(undefined, config1[prop]);
+  function defaultToConfig2(a, b) {
+    if (!utils.isUndefined(b)) {
+      return getMergedValue(undefined, b);
+    } else if (!utils.isUndefined(a)) {
+      return getMergedValue(undefined, a);
     }
   }
 
   // eslint-disable-next-line consistent-return
-  function mergeDirectKeys(prop) {
+  function mergeDirectKeys(a, b, prop) {
     if (prop in config2) {
-      return getMergedValue(config1[prop], config2[prop]);
+      return getMergedValue(a, b);
     } else if (prop in config1) {
-      return getMergedValue(undefined, config1[prop]);
+      return getMergedValue(undefined, a);
     }
   }
 
   const mergeMap = {
-    'url': valueFromConfig2,
-    'method': valueFromConfig2,
-    'data': valueFromConfig2,
-    'baseURL': defaultToConfig2,
-    'transformRequest': defaultToConfig2,
-    'transformResponse': defaultToConfig2,
-    'paramsSerializer': defaultToConfig2,
-    'timeout': defaultToConfig2,
-    'timeoutMessage': defaultToConfig2,
-    'withCredentials': defaultToConfig2,
-    'adapter': defaultToConfig2,
-    'responseType': defaultToConfig2,
-    'xsrfCookieName': defaultToConfig2,
-    'xsrfHeaderName': defaultToConfig2,
-    'onUploadProgress': defaultToConfig2,
-    'onDownloadProgress': defaultToConfig2,
-    'decompress': defaultToConfig2,
-    'maxContentLength': defaultToConfig2,
-    'maxBodyLength': defaultToConfig2,
-    'beforeRedirect': defaultToConfig2,
-    'transport': defaultToConfig2,
-    'httpAgent': defaultToConfig2,
-    'httpsAgent': defaultToConfig2,
-    'cancelToken': defaultToConfig2,
-    'socketPath': defaultToConfig2,
-    'responseEncoding': defaultToConfig2,
-    'validateStatus': mergeDirectKeys
+    url: valueFromConfig2,
+    method: valueFromConfig2,
+    data: valueFromConfig2,
+    baseURL: defaultToConfig2,
+    transformRequest: defaultToConfig2,
+    transformResponse: defaultToConfig2,
+    paramsSerializer: defaultToConfig2,
+    timeout: defaultToConfig2,
+    timeoutMessage: defaultToConfig2,
+    withCredentials: defaultToConfig2,
+    adapter: defaultToConfig2,
+    responseType: defaultToConfig2,
+    xsrfCookieName: defaultToConfig2,
+    xsrfHeaderName: defaultToConfig2,
+    onUploadProgress: defaultToConfig2,
+    onDownloadProgress: defaultToConfig2,
+    decompress: defaultToConfig2,
+    maxContentLength: defaultToConfig2,
+    maxBodyLength: defaultToConfig2,
+    beforeRedirect: defaultToConfig2,
+    transport: defaultToConfig2,
+    httpAgent: defaultToConfig2,
+    httpsAgent: defaultToConfig2,
+    cancelToken: defaultToConfig2,
+    socketPath: defaultToConfig2,
+    responseEncoding: defaultToConfig2,
+    validateStatus: mergeDirectKeys,
+    headers: (a, b) => mergeDeepProperties(headersToObject(a), headersToObject(b), true)
   };
 
   utils.forEach(Object.keys(config1).concat(Object.keys(config2)), function computeConfigValue(prop) {
     const merge = mergeMap[prop] || mergeDeepProperties;
-    const configValue = merge(prop);
+    const configValue = merge(config1[prop], config2[prop], prop);
     (utils.isUndefined(configValue) && merge !== mergeDirectKeys) || (config[prop] = configValue);
   });
 
diff --git a/lib/defaults/index.js b/lib/defaults/index.js
index b4b9db22bd..b68ab0eae6 100644
--- a/lib/defaults/index.js
+++ b/lib/defaults/index.js
@@ -10,7 +10,7 @@ import formDataToJSON from '../helpers/formDataToJSON.js';
 import adapters from '../adapters/index.js';
 
 const DEFAULT_CONTENT_TYPE = {
-  'Content-Type': 'application/x-www-form-urlencoded'
+  'Content-Type': undefined
 };
 
 /**
diff --git a/lib/utils.js b/lib/utils.js
index e075f9e2ab..849d0444b1 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -228,7 +228,7 @@ const trim = (str) => str.trim ?
  * @param {Function} fn The callback to invoke for each item
  *
  * @param {Boolean} [allOwnKeys = false]
- * @returns {void}
+ * @returns {any}
  */
 function forEach(obj, fn, {allOwnKeys = false} = {}) {
   // Don't bother if no value provided
@@ -263,6 +263,24 @@ function forEach(obj, fn, {allOwnKeys = false} = {}) {
   }
 }
 
+function findKey(obj, key) {
+  key = key.toLowerCase();
+  const keys = Object.keys(obj);
+  let i = keys.length;
+  let _key;
+  while (i-- > 0) {
+    _key = keys[i];
+    if (key === _key.toLowerCase()) {
+      return _key;
+    }
+  }
+  return null;
+}
+
+const _global = typeof self === "undefined" ? typeof global === "undefined" ? this : global : self;
+
+const isContextDefined = (context) => !isUndefined(context) && context !== _global;
+
 /**
  * Accepts varargs expecting each argument to be an object, then
  * immutably merges the properties of each object and returns result.
@@ -282,16 +300,18 @@ function forEach(obj, fn, {allOwnKeys = false} = {}) {
  * @returns {Object} Result of all merge properties
  */
 function merge(/* obj1, obj2, obj3, ... */) {
+  const {caseless} = isContextDefined(this) && this || {};
   const result = {};
   const assignValue = (val, key) => {
-    if (isPlainObject(result[key]) && isPlainObject(val)) {
-      result[key] = merge(result[key], val);
+    const targetKey = caseless && findKey(result, key) || key;
+    if (isPlainObject(result[targetKey]) && isPlainObject(val)) {
+      result[targetKey] = merge(result[targetKey], val);
     } else if (isPlainObject(val)) {
-      result[key] = merge({}, val);
+      result[targetKey] = merge({}, val);
     } else if (isArray(val)) {
-      result[key] = val.slice();
+      result[targetKey] = val.slice();
     } else {
-      result[key] = val;
+      result[targetKey] = val;
     }
   }
 
@@ -527,6 +547,11 @@ const reduceDescriptors = (obj, reducer) => {
 
 const freezeMethods = (obj) => {
   reduceDescriptors(obj, (descriptor, name) => {
+    // skip restricted props in strict mode
+    if (isFunction(obj) && ['arguments', 'caller', 'callee'].indexOf(name) !== -1) {
+      return false;
+    }
+
     const value = obj[name];
 
     if (!isFunction(value)) return;
@@ -540,7 +565,7 @@ const freezeMethods = (obj) => {
 
     if (!descriptor.set) {
       descriptor.set = () => {
-        throw Error('Can not read-only method \'' + name + '\'');
+        throw Error('Can not rewrite read-only method \'' + name + '\'');
       };
     }
   });
@@ -609,5 +634,8 @@ export default {
   toObjectSet,
   toCamelCase,
   noop,
-  toFiniteNumber
+  toFiniteNumber,
+  findKey,
+  global: _global,
+  isContextDefined
 };
diff --git a/test/module/test.js b/test/module/test.js
index 42c256b69b..166e649ce6 100644
--- a/test/module/test.js
+++ b/test/module/test.js
@@ -15,6 +15,7 @@ const __dirname = path.dirname(fileURLToPath(import.meta.url));
 const exec = util.promisify(cp.exec);
 
 const {Axios} = axiosFactory;
+
 const ignoreList = ['default'];
 
 const instance = axiosFactory.create({});
@@ -133,5 +134,4 @@ describe('module', function () {
       await exec(`npm test --prefix ${pkgPath}`, {});
     });
   });
-
 });
diff --git a/test/specs/core/mergeConfig.spec.js b/test/specs/core/mergeConfig.spec.js
index ffe666330e..2669b50923 100644
--- a/test/specs/core/mergeConfig.spec.js
+++ b/test/specs/core/mergeConfig.spec.js
@@ -1,5 +1,6 @@
 import defaults from '../../../lib/defaults';
 import mergeConfig from '../../../lib/core/mergeConfig';
+import {AxiosHeaders} from "../../../index.js";
 
 describe('core::mergeConfig', function() {
   it('should accept undefined for second argument', function() {
@@ -100,6 +101,27 @@ describe('core::mergeConfig', function() {
     expect(merged.nestedConfig.propertyOnRequestConfig).toEqual(true);
   });
 
+  describe('headers', ()=> {
+    it('should allow merging with AxiosHeaders instances', () => {
+      const merged = mergeConfig({
+        headers: new AxiosHeaders({
+          x: 1,
+          y: 2
+        })
+      }, {
+        headers: new AxiosHeaders({
+          X: 1,
+          Y: 2
+        })
+      });
+      expect(merged.headers).toEqual({
+        x: '1',
+        y: '2'
+      });
+    });
+  });
+
+
   describe('valueFromConfig2Keys', function() {
     const config1 = {url: '/foo', method: 'post', data: {a: 3}};
 
diff --git a/test/specs/headers.spec.js b/test/specs/headers.spec.js
index 92516c343d..765fd3c564 100644
--- a/test/specs/headers.spec.js
+++ b/test/specs/headers.spec.js
@@ -1,3 +1,5 @@
+const {AxiosHeaders} = axios;
+
 function testHeaderValue(headers, key, val) {
   let found = false;
 
@@ -106,12 +108,35 @@ describe('headers', function () {
     });
   });
 
-  it('should preserve content-type if data is false', function (done) {
+  it('should preserve content-type if data is false', async function () {
     axios.post('/foo', false);
 
-    getAjaxRequest().then(function (request) {
+    await getAjaxRequest().then(function (request) {
       testHeaderValue(request.requestHeaders, 'Content-Type', 'application/x-www-form-urlencoded');
-      done();
     });
   });
+
+  it('should allow an AxiosHeaders instance to be used as the value of the headers option', async ()=> {
+    const instance = axios.create({
+      headers: new AxiosHeaders({
+        xFoo: 'foo',
+        xBar: 'bar'
+      })
+    });
+
+    instance.get('/foo', {
+      headers: {
+        XFOO: 'foo2',
+        xBaz: 'baz'
+      }
+    });
+
+    await getAjaxRequest().then(function (request) {
+      expect(request.requestHeaders.xFoo).toEqual('foo2');
+      expect(request.requestHeaders.xBar).toEqual('bar');
+      expect(request.requestHeaders.xBaz).toEqual('baz');
+      expect(request.requestHeaders.XFOO).toEqual(undefined);
+    });
+
+  });
 });
diff --git a/test/specs/options.spec.js b/test/specs/options.spec.js
index f2e589b4da..9879fdc885 100644
--- a/test/specs/options.spec.js
+++ b/test/specs/options.spec.js
@@ -1,3 +1,5 @@
+import AxiosHeaders from "../../lib/core/AxiosHeaders.js";
+
 describe('options', function () {
   beforeEach(function () {
     jasmine.Ajax.install();
diff --git a/test/specs/utils/merge.spec.js b/test/specs/utils/merge.spec.js
index 6a820c6efe..75c6312dbf 100644
--- a/test/specs/utils/merge.spec.js
+++ b/test/specs/utils/merge.spec.js
@@ -83,4 +83,14 @@ describe('utils::merge', function () {
     expect(d).toEqual({a: [1, 2, 3]});
     expect(d.a).not.toBe(a);
   });
+
+  it('should support caseless option', ()=> {
+    const a = {x: 1};
+    const b = {X: 2};
+    const merged = merge.call({caseless: true}, a, b);
+
+    expect(merged).toEqual({
+      x: 2
+    });
+  });
 });
diff --git a/test/unit/adapters/http.js b/test/unit/adapters/http.js
index c92b0394bb..fba276d141 100644
--- a/test/unit/adapters/http.js
+++ b/test/unit/adapters/http.js
@@ -1393,6 +1393,7 @@ describe('supports http with nodejs', function () {
 
   it('should omit a user-agent if one is explicitly disclaimed', function (done) {
     server = http.createServer(function (req, res) {
+      console.log(req.headers);
       assert.equal("user-agent" in req.headers, false);
       assert.equal("User-Agent" in req.headers, false);
       res.end();
diff --git a/test/unit/core/AxiosHeaders.js b/test/unit/core/AxiosHeaders.js
index aa30091982..758a7d4cbb 100644
--- a/test/unit/core/AxiosHeaders.js
+++ b/test/unit/core/AxiosHeaders.js
@@ -33,7 +33,16 @@ describe('AxiosHeaders', function () {
 
       assert.strictEqual(headers.get('foo'), 'value1');
       assert.strictEqual(headers.get('bar'), 'value2');
-    })
+    });
+
+    it('should support adding multiple headers from raw headers string', function(){
+      const headers = new AxiosHeaders();
+
+      headers.set(`foo:value1\nbar:value2`);
+
+      assert.strictEqual(headers.get('foo'), 'value1');
+      assert.strictEqual(headers.get('bar'), 'value2');
+    });
 
     it('should not rewrite header the header if the value is false', function(){
       const headers = new AxiosHeaders();
@@ -338,4 +347,49 @@ describe('AxiosHeaders', function () {
       });
     });
   });
+
+  describe('AxiosHeaders.concat', function () {
+    it('should concatenate plain headers into an AxiosHeader instance', function () {
+      const a = {a: 1};
+      const b = {b: 2};
+      const c = {c: 3};
+      const headers = AxiosHeaders.concat(a, b, c);
+
+      assert.deepStrictEqual({...headers.toJSON()}, {
+        a: '1',
+        b: '2',
+        c: '3'
+      });
+    });
+
+    it('should concatenate raw headers into an AxiosHeader instance', function () {
+      const a = 'a:1\nb:2';
+      const b = 'c:3\nx:4';
+      const headers = AxiosHeaders.concat(a, b);
+
+      assert.deepStrictEqual({...headers.toJSON()}, {
+        a: '1',
+        b: '2',
+        c: '3',
+        x: '4'
+      });
+    });
+
+    it('should concatenate Axios headers into a new AxiosHeader instance', function () {
+      const a = new AxiosHeaders({x: 1});
+      const b = new AxiosHeaders({y: 2});
+      const headers = AxiosHeaders.concat(a, b);
+
+      assert.deepStrictEqual({...headers.toJSON()}, {
+        x: '1',
+        y: '2'
+      });
+    });
+  });
+
+  describe('toString', function () {
+    it('should serialize AxiosHeader instance to a raw headers string', function () {
+      assert.deepStrictEqual(new AxiosHeaders({x:1, y:2}).toString(), 'x: 1\ny: 2');
+    });
+  });
 });