fix bug: KeyError for description_format

[Brightcells]

redis.StrictRedis(connection_pool=redis.ConnectionPool(**connect_settings))

When connect_settings doesn't have key 'host', 'port', or 'db', will raise KeyError:

[gyermolenko]

it is hard to tell from description but if description_format from title is this one
than this is no longer an issue - all format arguments are passed to __init__ with defaults.

[andymccurdy]

I believe this had to do with the repr() of Connection/ConnectionPool/Redis instances which has been fixed since 3.0. Closing this for now. If there's still an issue with this, please feel free to reopen.