Assert session key is deleted from Redis.

Change name of test method as it is no longer just testing the dict
itself.
Modify assert to test for empty dict.
Add delete method to DummyRedis.
Modify invalidate() to delete key from Redis.
Removed repeat link
Merge pull request #34 from whiteroses/feature.delete_session_key_from_redis_on_invalidate

Delete session key from Redis on invalidate()
Merge pull request #35 from thapar/patch-1

Removed repeat link