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

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• Problem/bug report.
• Feature request.
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• 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.