Redis ClusterClient doesn't work with non default user

[gildaf]

(release 0.21.5)
The cluster client will fail to execute commands if the user is not "default" .

Reproduce:
setup a redis cluster and add an acl file to all nodes (helper script).
now connect to one node and execute a command

let client = redis::cluster::ClusterClient::open(vec!["redis://gil:somepass@localhost:5001"]).unwrap();
let mut connection = client.get_connection().unwrap();
let res: RedisResult<()> = connection.set("x", "3");

The set command will fail with Password authentication failed Error.

[jaymell]

Fixed by #596