Added support for abstract models that have other connections

This allows some models to have one connection while others have another connection.

for example models inheriting from ActiveRecord::Base can have one connection and models inheriting from OtherDBModel (abstract class) can be connected to a different DB.

I needed this for a project that had a connection for a bunch of models and another unrelated connection(db) for other models.

I wasn't sure if this project is maintained so I didn't write tests for this,but if it is not abandoned and think this fix is useful I will of course write some decent tests for this.

Thanks in advance.