Bugfix for eager loading has-many associations.

When eager loading has-many/has-one associations that specify a
custom primary_key, the relationship was ignoring that and using the
model's primary key value instead.
Merge pull request #10 from hpainter/has-many-eager-load-fix

Bugfix for eager loading has-many associations.
Merge pull request #11 from getloaded/has-many-eager-load-fix

Has many eager load fix