tests: fix generated test

Author: parthea

gapic/ads-templates/tests/unit/gapic/%name_%version/%sub/_test_mixins.py.j2

@@ -30,7 +30,11 @@ def test_{{ name|snake_case }}_rest(request_type):
         transport="rest",
     )
     request_init = {{ api.mixin_http_options["{}".format(name)][0].sample_request }}
-    request = request_type(**request_init)
+    if request_type is dict:
+        request = dict(request_init)
+    else:
+        # Native protobuf requires keyword arguments (**).
+        request = request_type(**request_init)
     # Mock the http request call within the method and fake a response.
     with mock.patch.object(type(client.transport._session), 'request') as req:
         # Designate an appropriate value for the returned response.