"Unrecognized field user" when trying to update project roles via REST

Steps to reproduce:

1. Install JIRA 5.2.8 and enable the remote API;
2. Create a new request using the following content:

   { "user" : ["admin"] }
   

   Note the request is according to the documentation.

3. Trigger it using 'PUT' method against the URL http://jira.domain.com/rest/api/2/project/PROJECT-KEY/role/ROLE-ID.

Current behavior:

1. JIRA will display the following error:

   {
       "errorMessages": [
           "Unrecognized field \"user\" (Class com.atlassian.jira.rest.v2.issue.project.ProjectRoleActorsUpdateBean), not marked as ignorable\n at [Source: org.apache.catalina.connector.CoyoteInputStream@4894b388; line: 1, column: 13] (through reference chain: com.atlassian.jira.rest.v2.issue.project.ProjectRoleActorsUpdateBean[\"user\"])"
       ]
   }
   

2. The user 'admin' was not added on the project role which has the ID 10001;

Expected behavior:

1. The user 'admin' should be updated on the project role which has the ID 10001;

Possible workaround:

1. The method 'POST' is working for the same URL.