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/utils.js

@@ -47,15 +47,6 @@ function isArrayBuffer(val) {
   return toString.call(val) === '[object ArrayBuffer]';
 }
 
-/**
- * Determine if a value is a FormData
- *
- * @param {Object} val The value to test
- * @returns {boolean} True if value is an FormData, otherwise false
- */
-function isFormData(val) {
-  return toString.call(val) === '[object FormData]';
-}
 
 /**
  * Determine if a value is a view on an ArrayBuffer
@@ -168,6 +159,21 @@ function isStream(val) {
   return isObject(val) && isFunction(val.pipe);
 }
 
+/**
+ * Determine if a value is a FormData
+ *
+ * @param {Object} thing The value to test
+ * @returns {boolean} True if value is an FormData, otherwise false
+ */
+function isFormData(thing) {
+  var pattern = '[object FormData]';
+  return thing && (
+    (typeof FormData === 'function' && thing instanceof FormData) ||
+    toString.call(thing) === pattern ||
+    (isFunction(thing.toString) && thing.toString() === pattern)
+  );
+}
+
 /**
  * Determine if a value is a URLSearchParams object
  *