include has_one relationships to avoid n+1 query when always_include_to_one_linkage_data is true

[barelyknown]

If always_include_to_one_linkage_data is true, we should include the has_one relationships in the query to avoid n+1 database requests.

[blmundie]

+1

[watsy0007]

+1

[robinw777]

This seems to be partially fixed as of v0.9.3, see https://github.com/cerebris/jsonapi-resources/blob/v0.9.3/lib/jsonapi/resource_serializer.rb#L458
However, there is a bug - it should check for relation.foreign_key instead of "#{relationship.name}_id", otherwise it will trigger n+1 query if the foreign_key is named differently than relationship name.