PR #1454 was merged with some missing fixes. All these fixes are
addressed by this commit:
- Rename ActiveModel::Serializer::Adapter::JsonApi::Association to
ActiveModel::Serializer::Adapter::JsonApi::Relationship
- Move ActiveModel::Serializer::Adapter:: JsonApi::Relationship and
ActiveModel::Serializer::Adapter::JsonApi::ResourceIdentifier to
ActiveModel::Serializer::Adapter::JsonApi::ApiObjects module
- Add unit test for
ActiveModel::Serializer::Adapter::JsonApi::Relationship
- Add unit test for
ActiveModel::Serializer::Adapter::JsonApi::ResourceIdentifier
