Don't perform unicode encoding/decoding if charset is None.

If RedisProtocol has a charset set to None, it shouldn't try to decode received strings. 

When client receives binary data, sometimes it may look like valid utf-8 (or other encoding), and in this case it can be decoded and unicode object is returned. This causes output ambiguity and some performance impact.

Thus I propose to turn off the encoding/decoding via setting charset=None for the protocol.