Agreed this is poorly named. It pre-dates this PR but I think the poor name actually git blames to me anyway.

The value for `default_db` actually comes from the URI parser and its never a db instance, it's always a DB name (string). This is just an internal instance variable storing what we parsed from the URI so we can actually rename this to whatever we want without breaking anyone.

I'm inclined to call this @default_db_name (exactly what it is) but that makes `DEFAULT_DB_NAME` confusing. Or is that fine?