Change ResourceIdentity <=> to take resource_klass into account when sorting. #1429

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

I propose we compare the resource_klass.name first and then the ids. This would result in grouping of identities by type before ids. Since any sorting of these does not affect compliance with the spec I consider this a minor issue for polymorphic arrays of related identities.