Used instanceof to determine FormData in utils.isFormData. Fix #323.

lib/utils.js

@@ -33,7 +33,7 @@ function isArrayBuffer(val) {
  * @returns {boolean} True if value is an FormData, otherwise false
  */
 function isFormData(val) {
-  return toString.call(val) === '[object FormData]';
+  return (typeof FormData !== 'undefined') && (val instanceof FormData);
 }
 
 /**