Modifying isURLSearchParams function to use instanceof instead of duck typing

lib/utils.js

@@ -149,15 +149,7 @@ function isStream(val) {
  * @returns {boolean} True if value is a URLSearchParams object, otherwise false
  */
 function isURLSearchParams(val) {
-  // Object.prototype.toString will return [object Object] for a polyfill
-  // Hence, we have to use duck typing
-  return toString.call(val) === '[object URLSearchParams]' || (
-    isObject(val) &&
-    isFunction(val.append) &&
-    isFunction(val.delete) &&
-    isFunction(val.get) &&
-    isFunction(val.set)
-  );
+  return typeof URLSearchParams !== 'undefined' && val instanceof URLSearchParams;
 }
 
 /**