/rest/api/2/mypreferences does not respect Accept and Content-Type headers

Issue Summary

This is reproducible on Data Center: (yes)

Currently when calling /rest/api/2/mypreferences?key=jira.user.locale endpoint, the results are sent and returned as a String regardless of the Accept: or Content-type: headers.

Using Content-type: text/plain generates a HTTP 415 Unsupported Media Type response.

Steps to Reproduce

1. Using Jira 9.9.0 make a PUT request with

   curl --silent --verbose --user admin:admin --request PUT --url 'http://localhost:2990/jira/rest/api/2/mypreferences?key=jira.user.locale' --header 'Content-type: application/json' --data-raw 'en_AU'
   

1. Make a GET request

   curl --silent --verbose --user admin:admin --request GET --url 'http://localhost:2990/jira/rest/api/2/mypreferences?key=jira.user.locale' --header 'Accept: application/json'
   

Expected Results

And error should be thrown because the PUT payload and the GET results should be in JSON format.

Actual Results

The PUT response has no error for a raw String payload

HTTP/1.1 204
...
Content-Type: application/json;charset=UTF-8

and the GET request has the payload as a raw String even though we stated Accept: application/json.

HTTP/1.1 200
...
Content-Type: application/json;charset=UTF-8

en_AU

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available.