REST API for "/group/membership" returns XML response when JSON response is requested

Steps to Reproduce

1. Set up a GET JSON call with no request body to REST API: http://<crowd-url>/crowd/rest/usermanagement/latest/group/membership
2. Ensure that the response data type is in JSON

Expected Results

The response should be in JSON, which contains full details of all group memberships, with users and nested groups.

Actual Results

The response is in XML, whether or not we specify the response data type.

Findings

When trying a GET JSON call to /user?username=<user>, it is able to return in either JSON or XML