Added the ability for the url-encoded-form serializer to respect the formSerializer config; (#4721)

Added test for `formSerializer` config in context of `url-encoded-form` serializer;

lib/defaults/index.js

@@ -77,7 +77,7 @@ var defaults = {
 
     if (isObjectPayload) {
       if (contentType.indexOf('application/x-www-form-urlencoded') !== -1) {
-        return toURLEncodedForm(data).toString();
+        return toURLEncodedForm(data, this.formSerializer).toString();
       }
 
       if ((isFileList = utils.isFileList(data)) || contentType.indexOf('multipart/form-data') !== -1) {

lib/helpers/toURLEncodedForm.js

@@ -4,8 +4,8 @@ var utils = require('../utils');
 var toFormData = require('./toFormData');
 var platform = require('../platform/');
 
-module.exports = function toURLEncodedForm(data) {
-  return toFormData(data, new platform.classes.URLSearchParams(), {
+module.exports = function toURLEncodedForm(data, options) {
+  return toFormData(data, new platform.classes.URLSearchParams(), Object.assign({
     visitor: function(value, key, path, helpers) {
       if (platform.isNode && utils.isBuffer(value)) {
         this.append(key, value.toString('base64'));
@@ -14,5 +14,5 @@ module.exports = function toURLEncodedForm(data) {
 
       return helpers.defaultVisitor.apply(this, arguments);
     }
-  });
+  }, options));
 };

lib/platform/browser/classes/URLSearchParams.js

@@ -1,8 +1,6 @@
 'use strict';
 
-module.exports = (function getURLSearchParams(nativeURLSearchParams) {
-  if (typeof nativeURLSearchParams === 'function') return nativeURLSearchParams;
-
+module.exports = typeof URLSearchParams !== 'undefined' ? URLSearchParams : (function defineURLSearchParams() {
   function encode(str) {
     var charMap = {
       '!': '%21',
@@ -35,4 +33,4 @@ module.exports = (function getURLSearchParams(nativeURLSearchParams) {
   };
 
   return URLSearchParams;
-})(URLSearchParams);
+})();