Fix hasMany record set returned from `with` key on a finder.

The record set built from records read from a hasMany relationship should use the related model, not the original model. If the original model is used, the record set is indexed by the original model's key. Since this is a hasMany relation, the original model's key is set on each related record, and thus all related records will share the same index in the record set. Using the related model indexes the record set by the related model's key, which (I think is the desired behavior).