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

Start by reviewing commit 010f024758a41460ebe51f13b35e0b3136a95e79 and the resource-creation path it changed. Reproduce creation under v0.9.12, compare the self-link key handling with v0.9.0, and verify that the response includes the expected Location header.