Added saving url to File's "_to_native"

parse_rest/datatypes.py

@@ -201,7 +201,8 @@ class File(ParseType):
     def _to_native(self):
         return {
             '__type': 'File',
-            'name': self._name
+            'name': self._name,
+            'url': self._file_url
             }
 
     url = property(lambda self: self._file_url)
@@ -388,4 +389,4 @@ class Object(ParseResource):
                 }
             }
         self.__class__.PUT(self._absolute_url, **payload)
-        self.__dict__[key] = ''
\ No newline at end of file
+        self.__dict__[key] = ''