Fixes https://github.com/mongoid/moped/issues/10.
Thanks to @mikezter.

The Problem was that the configuration contained only the uri and not the database and hosts. That worked initially, but when calling #mongo_session(https://github.com/i0rek/mongoid/blob/uri_fix/lib/mongoid/sessions.rb) looking up the database name doesn't work because its not there.

Edit: I'm aware that I'm modifying the configuration. Not sure what to do about it.