Use ParseObject::create() to create new ParseUser

Should fix thrown exception mentioned in parse-community#128

src/Parse/ParseUser.php

@@ -177,7 +177,7 @@ public static function logInWithFacebook($id, $access_token, $expiration_date =
             ]
         ]];
         $result = ParseClient::_request("POST", "/1/users", "", json_encode($data));
-        $user = new ParseUser();
+        $user = ParseObject::create('_User');
         $user->_mergeAfterFetch($result);
         $user->handleSaveResult(true);
         ParseClient::getStorage()->set("user", $user);