Update resource links to behave consistently

This fixes the inconsistency of resource links by association type, making has_many behave the same as has_many :through and belongs_to. With this code change, all resource-embedded links are shown automatically. The "linked" section remains unpopulated unless the associated resource is included via params.

This would resolve [Issue #82](https://github.com/RestPack/restpack_serializer/issues/82): Inconsistent behavior with has_many and belong_to links.