Fix default_sort for show_related_resources

[nickgonzales]

The default_sort support from #756 does not work when querying for related resources. I searched through the source and found RelationshipBuilder::build_to_many has the following logic:

sort_criteria = options.fetch(:sort_criteria, {})
unless sort_criteria.nil? || sort_criteria.empty?
  order_options = relationship.resource_klass.construct_order_options(sort_criteria)
  records = resource_klass.apply_sort(records, order_options, @context)
end

The unless block shouldn't be there. It should be the resource's responsibility to apply sorts, or, if they're null, apply the default sort:

sort_criteria = options.fetch(:sort_criteria, {})
order_options = relationship.resource_klass.construct_order_options(sort_criteria)
records = resource_klass.apply_sort(records, order_options, @context)

This method looks totally different on the 0.9 branch, so I don't know if the issue exists there.

[lgebhardt]

@nickgonzales Thanks, the issue did exist on 0.9.