Patching {"data": <anything>} on To-One relationship fails

[tpansino]

Describe the bug
This is REALLY similar to #46, but the fix is totally different...

See https://jsonapi.org/format/#crud-updating-to-one-relationships

Specifically, this case:

PATCH /articles/1/relationships/author HTTP/1.1
Content-Type: application/vnd.api+json
Accept: application/vnd.api+json

{
  "data": { "type": "people", "id": "12" }
}

Isn't working properly.

This is a regression created in #29 by removing this line: https://github.com/Vacasa/drf-jsonapi/pull/32/files#diff-9a2180fee48df47db205fc59be17fa76L380

Without that, the change to the To-One relationship never gets saved and committed to the database.

To Reproduce
Define two models, one with a ForeignKey linking it to the other, and two corresponding endpoints with a relationship endpoint between the resources. Attempt to PATCH the To-One direction of the relationship with a related resource. Observe no error, but the relationship does not change.

Expected behavior
The response should be a 204 No Content code with the relationship successfully changed.

[tpansino]

I made a mistake in #54 which prevented models with multiple required To-One relationships from being saved to the DB. Resolved in #56 by putting the save() call in the right place.

[tpansino]

Just realized we still need to fix this for master, reopening.