Make #connected? check the connection flags.

'#connected?' is broken, it should return 'false' if the server kicks
the client (due to server-side timeout).

Bitwise AND (&) the connection flags against REDIS_CONNECTED.
https://github.com/redis/hiredis/blob/master/hiredis.h#L61

This didn't work as I expected. I suppose the client socket doesn't know the server-side closed.