Redis backend connection fix

[kirberich]

Sorry again about my messing about with unrelated commits, this is a copy of the previous PR with the correct commits this time!

Hi!

As mentioned yesterday in #149, I found some problems with cachecontrol attempting to close the connection to redis when .close is called on the cache adapter, causing issues because the Redis object doesn't have a disconnect method.

I'm not an expert on redis, but as far as I can tell py-redis uses connection pooling by default, making it unnecessary to explicitly close connections (source: http://stackoverflow.com/questions/24875806/redis-in-python-how-do-you-close-the-connection)

I've coded up a quick fix that simply doesn't do anything when closing a redis cache, which I think is the correct behaviour. There might be cases where a user manages to instantiate redis without a connection pool, in which case closing the connection might be necessary, but I don't even know how to do that, so I'm hoping we can shelve that potential problem for now.

For reference, here's a quick example of code I've been using that caused problems for me with the original behaviour:

@contextmanager
def cache_session(session=None):
    """Context manager to easily apply caching to a session.

    An existing session can be passed in, otherwise
    a new Session is created.
    """

    session = session if session else Session()
    session.mount('http://', cache_adapter)
    session.mount('https://', cache_adapter)

    yield session