JsonAPI fails to create a resource on a POST request when the entity has a uuid entity key present

To better demonstrate the issue, I've created a test module called jsonapi_test_custom_entity_uuid which alters the entity_test uuid key to 'test_uuid' and created a test EntityCustomUuidKeyTest which extends EntityTestTest and just enables this new module which now fails on the ::testIndividual() test function.

Attached is both a patch with both the tests and the fix which passes tests and a patch with just the tests which fail.

Cheers

I really appreciate the fact you made tests to proove the issue. This helps a lot! Thank you.