Fix RedisCache.close() to check for client before closing.

[wloggains-mmllc]

Problem
There are some situations where the _client in RedisCache is not instantiated when the close() method is called. We have ran into this specifically with django.core.cache.CacheHandler.close_caches(). While it does not cause an error, it is an inefficiency.

Solution
This pull request makes an update to django_redis/cache.py::RedisCache.close() to prevent unnecessary initialization of the _client.

The close method now checks if _client is not None before attempting to close the client.

I have also added a test.

[kingbuzzman]

@WisdomPill hey is there anything else you'd like to see?