Fix issue with relationship sorts due to missing join_manager

[lgebhardt]

Fix issue with relationship sorts due to missing join_manager.

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• [ ] Maintains compliance with JSON:API
• [ ] Adequate test coverage exists to prevent regressions

[lgebhardt]

Locally all the tests pass with the JSONAPI::ActiveRelationRetrievalV09, except MultipleActiveRelationResourceTest#test_correct_resource_retrieval_strategy which is expected.

[lgebhardt]

I agree we need tests for the different retrieval strategies. I plan to get them into the testing matrix soon.