Fix tests to pass with recent GAE SDK versions

This fixes tests which are currently failing at HEAD when running against GAE SDK v1.9.11.

```
======================================================================
FAIL: test_known_user (gaeauth.tests.middleware.GoogleOAuthRemoteUserMiddlewareTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/mmcdonald/src/django-gaeauth/gaeauth/tests/middleware.py", line 61, in test_known_user
    self.assertEqual(response.context['user'].username, 'user')
AssertionError: 'example' != 'user'

======================================================================
FAIL: test_no_google_user (gaeauth.tests.middleware.GoogleOAuthRemoteUserMiddlewareTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/mmcdonald/src/django-gaeauth/gaeauth/tests/middleware.py", line 85, in test_no_google_user
    self.assertTrue(response.context['user'].is_anonymous())
AssertionError: False is not true

======================================================================
FAIL: test_unknown_user (gaeauth.tests.middleware.GoogleOAuthRemoteUserMiddlewareTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/google/home/mmcdonald/src/django-gaeauth/gaeauth/tests/middleware.py", line 46, in test_unknown_user
    self.assertEqual(response.context['user'].username, 'user')
AssertionError: 'example' != 'user'

----------------------------------------------------------------------
Ran 19 tests in 0.129s

FAILED (failures=3)
Destroying test database for alias 'default'...
```