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

Inizia esaminando il commit 010f024758a41460ebe51f13b35e0b3136a95e79 e il percorso di creazione delle risorse che ha modificato. Riproduci la creazione con v0.9.12, confronta la gestione della chiave self-link con v0.9.0 e verifica che la risposta includa l’header Location previsto.