Resolved an issue with the Mongoid adapter + Mongoid 2.0 RC

I ran into a problem with Pickle when using Mongoid 2.0.0.rc.8 .  Mongoid::Document::PickleAdapter.model_classes assumes that all descendants of Mongoid::Document will be a model, but Mongoid 2.0 creates an empty nameless class that includes Mongoid::Document inside Mongoid::Config#destructive_fields.  This nameless instance causes a NilClass exception inside model_classes.

I didn't see any tests for Mongoid in your suite (please let me know if I missed them), so I did not include a new test for this situation.  The change is small and innocuous, though, so a test may not be necessary.

Thanks for all of your great work on Pickle,
-Matt Royal