Decoding data is not nesessary on some Redis configurations

What reason for this line here?
https://github.com/aio-libs/aiohttp-session/blob/606cacafc47383f99cff39aba31f20bb5db3b879/aiohttp_session/redis_storage.py#L53

I want to store some bytes in session. Changed encoder ` setup_session(app, RedisStorage(redis_pool, encoder=pickle.dumps, decoder=pickle.loads))`. And I have error on getting session:
```

data = data.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid start byte

```
It also breakes `msgpack`