good catch. but maybe this shouldn't actually change?

originally we were setting this to `opts[:default_db] || DEFAULT_DB_NAME` here in initialize, but I moved that to `MongoClient#DB` because I needed to check if `default_db` was unassigned elsewhere. Always giving this a value in initialize was prohibiting me from checking that so I moved DEFAULT_DB_NAME to where it was actually being used.

I did forget to revert this when I removed my changes to default db but I actually think it kind of makes sense to leave this unassigned and put the logic to fallback to `DEFAULT_DB_NAME` only in `MongoClient#DB`. The value of `default_db` comes from the URI parser and I think its handy to know when that hasn't been set.