text: Title: ping function call returns PONG when Redis server is not running (sock connection).
  Body: ```
  import aioredis
  conn = await aioredis.create_redis_pool('unix:///var/run/redis/redis-server.sock?db=0')
  await conn.ping()
  ```
  
  returns **b'PONG'** even though the server is not running.
  
  ```
  import redis
  r = redis.Redis('unix:///var/run/redis/redis-server.sock?db=0')
  r.ping()
  ```
  
  raises the **ConnectionError** exception.
  
  aioredis should not return **b'PONG'** on its own, it should only come from a Redis server.
  
  Maybe `self._process_data(data or b'PONG')` (aioredis/connection.py) is the culprit?