Location header fails to be set in v0.9.12

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Description

When upgrading from jsonapi-resources v0.9.0 to v0.9.12 we've noticed a regression with the Location header being set when creating resources.

On further investigation, it appears this change introduced string keys for referencing self links but did not update the setting of the location header accordingly: https://github.com/cerebris/jsonapi-resources/commit/010f024758a41460ebe51f13b35e0b3136a95e79

Beginne mit der Überprüfung des Commits 010f024758a41460ebe51f13b35e0b3136a95e79 und des von ihm geänderten Pfads zur Ressourcenerstellung. Reproduziere die Erstellung unter v0.9.12, vergleiche die Handhabung des self-link-Schlüssels mit v0.9.0 und überprüfe, dass die Antwort den erwarteten Location-Header enthält.