Groups related by pivot's id on belongsTo through relation.

When the relation is `belongsTo through` on eager loading,
groups related models by pibot's id.
This would make it possible to fetch the related model's data
by eager loading on `belongsTo through` relation and
fixes issue #214.
Merge pull request #215 from p-baleine/topic-belongsTo-through

Groups related by pivot's id on belongsTo through relation.