'Test parameters filtering based on the prefix.
With ``assertion_prefix`` default value set to empty string
issue an unscoped token from assertion EMPLOYEE_ASSERTION.
Next, configure ``assertion_prefix`` to value ``UserName``.
Try issuing unscoped token with EMPLOYEE_ASSERTION.
Expect server to raise exception.Unathorized exception.'

def test_assertion_prefix_parameter_expect_fail(self):

    r = self._issue_unscoped_token()
    self.assertIsNotNone(r.headers.get('X-Subject-Token'))
    self.config_fixture.config(group='federation', assertion_prefix='UserName')
    self.assertRaises(exception.Unauthorized, self._issue_unscoped_token)
