As a Wikidata PM I want clients of REST API know the allowed content types so that they can make valid PATCH requests.
Currently the allowed content type is application/json-patch+json.
When returning 415 Unsupported Content-Type response it should include Accept-Patch: application/json-patch+json header.
While application/json is an accepted content-type (API assuming input is application/json-patch+json and treats it as such), the API should be clearly promoting usage of application/json-patch+json. Therefore PATCH requests of content-type application/json should also result in responses containing Accept-Patch: application/json-patch+json header even if the request is processed successfully.