Fix RBAC unit test failure due to uninitialized mock.

Initialized get_param() call to return None for all parameters.

Change-Id: Ic672899a34575407f2f30e0876a71693fdc15864

barbican/tests/api/test_resources_policy.py

@@ -51,6 +51,7 @@ class BaseTestCase(unittest.TestCase):
     def _generate_req(self, roles=None, accept=None):
         """Generate a fake HTTP request with security context added to it."""
         req = mock.MagicMock()
+        req.get_param.return_value = None
 
         kwargs = {
             'user': None,