Fixing usage of http= vs. _http= in core unit test. (googleapis#3243)

Issue caused by two unrelated PRs merging in close
proximity: googleapis#3235 and googleapis#3230.

Author: dhermes

core/tests/unit/test_client.py

@@ -53,7 +53,7 @@ def test_unpickleable(self):
         CREDENTIALS = _make_credentials()
         HTTP = object()
 
-        client_obj = self._make_one(credentials=CREDENTIALS, http=HTTP)
+        client_obj = self._make_one(credentials=CREDENTIALS, _http=HTTP)
         with self.assertRaises(pickle.PicklingError):
             pickle.dumps(client_obj)