Fixed isFormData predicate; (#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 through `form-data` package;

lib/helpers/toFormData.js

@@ -1,5 +1,7 @@
 'use strict';
 
+var utils = require('../utils');
+
 function combinedKey(parentKey, elKey) {
   return parentKey + '.' + elKey;
 }
@@ -11,7 +13,7 @@ function buildFormData(formData, data, parentKey) {
     });
   } else if (
     typeof data === 'object' &&
-    !(data instanceof File || data === null)
+    !(utils.isFile(data) || data === null)
   ) {
     Object.keys(data).forEach(function buildObject(key) {
       buildFormData(
@@ -44,10 +46,12 @@ function buildFormData(formData, data, parentKey) {
  * type FormVal = FormDataNest | FormDataPrimitive
  *
  * @param {FormVal} data
+ * @param {?Object} formData
  */
 
-module.exports = function getFormData(data) {
-  var formData = new FormData();
+module.exports = function getFormData(data, formData) {
+  // eslint-disable-next-line no-param-reassign
+  formData = formData || new FormData();
 
   buildFormData(formData, data);