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/defaults.js

@@ -3,6 +3,7 @@
 var utils = require('./utils');
 var normalizeHeaderName = require('./helpers/normalizeHeaderName');
 var enhanceError = require('./core/enhanceError');
+var toFormData = require('./helpers/toFormData');
 
 var DEFAULT_CONTENT_TYPE = {
   'Content-Type': 'application/x-www-form-urlencoded'
@@ -71,10 +72,17 @@ var defaults = {
       setContentTypeIfUnset(headers, 'application/x-www-form-urlencoded;charset=utf-8');
       return data.toString();
     }
-    if (utils.isObject(data) || (headers && headers['Content-Type'] === 'application/json')) {
+
+    var isObjectPayload = utils.isObject(data);
+    var contentType = headers && headers['Content-Type'];
+
+    if ( isObjectPayload && contentType === 'multipart/form-data' ) {
+      return toFormData(data,  new (this.env && this.env.FormData || FormData));
+    } else if ( isObjectPayload || contentType === 'application/json' ) {
       setContentTypeIfUnset(headers, 'application/json');
       return stringifySafely(data);
     }
+
     return data;
   }],
 
@@ -112,6 +120,8 @@ var defaults = {
   maxContentLength: -1,
   maxBodyLength: -1,
 
+  env: {},
+
   validateStatus: function validateStatus(status) {
     return status >= 200 && status < 300;
   },