Modifying isURLSearchParams function to use instanceof instead of duck typing

test/specs/__helpers.js

@@ -1,6 +1,9 @@
 // Polyfill ES6 Promise
 require('es6-promise').polyfill();
 
+// Polyfill URLSearchParams
+URLSearchParams = require('url-search-params');
+
 // Import axios
 axios = require('../../index');
 

test/specs/requests.spec.js

@@ -1,5 +1,3 @@
-var URLSearchParams = require('url-search-params');
-
 describe('requests', function () {
   beforeEach(function () {
     jasmine.Ajax.install();

test/specs/utils/isX.spec.js

@@ -1,6 +1,5 @@
 var utils = require('../../../lib/utils');
 var Stream = require('stream');
-var URLSearchParams = require('url-search-params');
 
 describe('utils::isX', function () {
   it('should validate Array', function () {