Revert "redisBufferRead: Clear REDIS_CONNECTED flag when server closed connection"

This reverts commit 1db17f25.

If the `REDIS_CONNECTED` flag is cleared,
the async onDisconnect callback function will never be called.
This causes problems as the disconnect is never reported back to the user.

Closes #359

hiredis.c

@@ -808,7 +808,6 @@ int redisBufferRead(redisContext *c) {
             return REDIS_ERR;
         }
     } else if (nread == 0) {
-        c->flags &= ~REDIS_CONNECTED;
         __redisSetError(c,REDIS_ERR_EOF,"Server closed the connection");
         return REDIS_ERR;
     } else {