Linked data with polymorphism does not work with custom `key_type`

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

If you create a polymorphic association with custom key_type id is always taken which leads to a wrong data sharing.

Bug reports:

Everything is on lib/jsonapi/resource_serializer.rb:464
When calling source.public_send("#{relationship.name}_id"), it is fine (faster) but it does not take care about custom key_type.
But if you call source.public_send(relationship.name).try(:id), you're using the custom key_type.

Features:

We can set another argument to has_one to be able to mention that the key_type has been modified.
Here is my proposition:

# app/resources/my_awesome_resource
has_one :positionable, polymorphic: true, foreign_key: :positionable_id, foreign_key_type_changed: true

# jsonapi/resource_serializer.rb

  def foreign_key_value(source, relationship)
      # If you have changed the key_name, don't even try to look at `"#{relationship.name}_id"` 
      # just load the association and call the custom key_name
      foreign_key_type_changed = relationship.options[:foreign_key_type_changed] || false
      related_resource_id =
        if source.preloaded_fragments.has_key?(format_key(relationship.name))
          source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
        elsif !foreign_key_type_changed && source.respond_to?("#{relationship.name}_id")
          # If you have direct access to the underlying id, you don't have to load the relationship
          # which can save quite a lot of time when loading a lot of data.
          # This does not apply to e.g. has_one :through relationships.
          source.public_send("#{relationship.name}_id")
        else
          source.public_send(relationship.name).try(:id)
        end
      return nil unless related_resource_id
      @id_formatter.format(related_resource_id)
    end

Start at lib/jsonapi/resource_serializer.rb:464 and inspect foreign_key_value, focusing on how polymorphic relationships obtain their ids when a custom key_type is configured. Reproduce the reported case and add coverage showing that the serialized relationship id respects the custom key_type rather than always using the direct association id.