text: Title: Cleanup code from TODOs
  Body: Cleanup the following TODOs:
  
  * [x] https://github.com/aio-libs/aioredis/blob/master/aioredis/commands/server.py#L223
  * [x] https://github.com/aio-libs/aioredis/blob/master/aioredis/commands/generic.py#L146
      pass `encoding='utf-8'` parameter to command.