make user-agent and content-type overwritable with undefined

lib/request.js

@@ -104,12 +104,24 @@ function Request (options) {
   const headers = options.headers || {}
 
   for (const field in headers) {
+    const fieldLowerCase = field.toLowerCase()
+    if (
+      (
+        fieldLowerCase === 'user-agent' ||
+        fieldLowerCase === 'content-type'
+      ) && headers[field] === undefined
+    ) {
+      this.headers[fieldLowerCase] = undefined
+      continue
+    }
     const value = headers[field]
     assert(value !== undefined, 'invalid value "undefined" for header ' + field)
-    this.headers[field.toLowerCase()] = '' + value
+    this.headers[fieldLowerCase] = '' + value
   }
 
-  this.headers['user-agent'] = this.headers['user-agent'] || 'lightMyRequest'
+  if (('user-agent' in this.headers) === false) {
+    this.headers['user-agent'] = 'lightMyRequest'
+  }
   this.headers.host = this.headers.host || options.authority || hostHeaderFromURL(parsedURL)
 
   if (options.cookies) {
@@ -143,7 +155,10 @@ function Request (options) {
 
   if (payload && typeof payload !== 'string' && !payloadResume && !Buffer.isBuffer(payload)) {
     payload = JSON.stringify(payload)
-    this.headers['content-type'] = this.headers['content-type'] || 'application/json'
+
+    if (('content-type' in this.headers) === false) {
+      this.headers['content-type'] = 'application/json'
+    }
   }
 
   // Set the content-length for the corresponding payload if none set

test/test.js

@@ -1924,3 +1924,28 @@ test('should work with pipeline', (t) => {
     t.equal(res.rawPayload.toString(), 'example.com:8080|/hello')
   })
 })
+
+test('should leave the headers user-agent and content-type undefined when the headers are explicitly set to undefined in the inject', (t) => {
+  t.plan(5)
+  const dispatch = function (req, res) {
+    t.ok(Array.isArray(req.rawHeaders))
+    t.equal(req.headers['user-agent'], undefined)
+    t.equal(req.headers['content-type'], undefined)
+    t.equal(req.headers['x-foo'], 'bar')
+    res.writeHead(200, { 'Content-Type': 'text/plain' })
+    res.end('Ok')
+  }
+
+  inject(dispatch, {
+    url: 'http://example.com:8080/hello',
+    method: 'POST',
+    headers: {
+      'x-foo': 'bar',
+      'user-agent': undefined,
+      'content-type': undefined
+    },
+    body: {}
+  }, (err, res) => {
+    t.error(err)
+  })
+})