Always incldue redis-db in client description

We operate in an environment with multiple databases per redis cluster. While developing and in the production console it is useful to see what database we are currently connected to.

With this patch, calls to "inspect" on "RedisFailover::Client" instances will include the database in the description. I was on the fence about omitting the db when it was nil/zero but decided to keep it in (I went with a hybrid of the "redis-rb" and "redis-cli" output).

I also created a new test in the "client_spec" for this change.