_Note to self_ 

The reason for this change isn't clear. Removing it makes a test fail:

    ======================================================================
    FAIL: test_non_default_cache (django.contrib.sessions.tests.CacheSessionTests)
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/Users/myk/Documents/dev/django/django/test/utils.py", line 217, in inner
        return test_func(*args, **kwargs)
      File "/Users/myk/Documents/dev/django/django/contrib/sessions/tests.py", line 500, in test_non_default_cache
        self.assertEqual(caches['default'].get(self.session.cache_key), None)
    AssertionError: {} != None