Fixed isFormData predicate; (axios#4413)

Added support for automatic object serialization to FormData if `Content-Type` is `multipart/form-data`;
Added support for FormData to be overloaded using `config.env.FormData` option;
Added support for FormData in node.js environment using `form-data` package;

(cherry picked from commit 73e3bdb)

index.d.ts

@@ -107,6 +107,9 @@ export interface AxiosRequestConfig<D = any> {
   transitional?: TransitionalOptions;
   signal?: AbortSignal;
   insecureHTTPParser?: boolean;
+  env?: {
+    FormData?: new (...args: any[]) => object;
+  };
 }
 
 export interface HeadersDefaults {

lib/utils.js

@@ -62,15 +62,6 @@ function isBuffer(val) {
  */
 var isArrayBuffer = kindOfTest('ArrayBuffer');
 
-/**
- * Determine if a value is a FormData
- *
- * @param {Object} val The value to test
- * @returns {boolean} True if value is an FormData, otherwise false
- */
-function isFormData(val) {
-  return toString.call(val) === '[object FormData]';
-}
 
 /**
  * Determine if a value is a view on an ArrayBuffer
@@ -180,6 +171,21 @@ function isStream(val) {
   return isObject(val) && isFunction(val.pipe);
 }
 
+/**
+ * Determine if a value is a FormData
+ *
+ * @param {Object} thing The value to test
+ * @returns {boolean} True if value is an FormData, otherwise false
+ */
+function isFormData(thing) {
+  var pattern = '[object FormData]';
+  return thing && (
+    (typeof FormData === 'function' && thing instanceof FormData) ||
+    toString.call(thing) === pattern ||
+    (isFunction(thing.toString) && thing.toString() === pattern)
+  );
+}
+
 /**
  * Determine if a value is a URLSearchParams object
  * @function