Pass kwargs through URL to superclass

Fixes #702
See #701

flask_restful/fields.py

@@ -294,8 +294,8 @@ class Url(Raw):
     :param scheme: URL scheme specifier (e.g. ``http``, ``https``)
     :type scheme: str
     """
-    def __init__(self, endpoint=None, absolute=False, scheme=None):
-        super(Url, self).__init__()
+    def __init__(self, endpoint=None, absolute=False, scheme=None, **kwargs):
+        super(Url, self).__init__(**kwargs)
         self.endpoint = endpoint
         self.absolute = absolute
         self.scheme = scheme

tests/test_fields.py

@@ -280,6 +280,14 @@ def test_url_with_blueprint_absolute_scheme(self):
         with app.test_request_context("/foo/hey", base_url="http://localhost"):
             self.assertEquals("https://localhost/foo/3", field.output("hey", Foo()))
 
+    def test_url_superclass_kwargs(self):
+        app = Flask(__name__)
+        app.add_url_rule("/<hey>", "foobar", view_func=lambda x: x)
+        field = fields.Url(absolute=True, attribute='hey')
+
+        with app.test_request_context("/hey"):
+            self.assertEquals("http://localhost/3", field.output("hey", Foo()))
+
     def test_int(self):
         field = fields.Integer()
         self.assertEquals(3, field.output("hey", {'hey': 3}))