fix admin logout bug

[leavest]

#207

[mingchen]

Can you fix the failed test ?

[leavest]

Can you fix the failed test ?

it is the problem for the test case.
in test_backend.py line 387

    with pytest.raises(ImproperlyConfigured) as excinfo:
        user = backend.authenticate(
            request, ticket='fake-ticket', service='fake-service',
        )

    assert "CAS_CREATE_USER_WITH_ID is True, but `'id'` is not part of attributes." in str(excinfo)

see pytest-dev/pytest#5413.
in newest pytest, str(excinfo) returrn ExceptionInfo.repr_(), like <ExceptionInfo ImproperlyConfigured tblen=3>
but not error value for Exception, like "CAS_CREATE_USER_WITH_ID is True, but `'id'` is not part of attributes."