Update owninguser of a task

Hi!

I want to update a task with a new owninguser using REST API, the update request receives 204 response that everything should be OK, but the value _owninguser_value is not updated. Is this a bug? We are using version CRM version 8.1. How can I change owner of a task?

PATCH[organization URI]/api/data/v8.1/tasks(df50766a-0146-49bb-8684-35f26b91dd6c)
Content-Type: application/json; charset=utf-8
Accept: application/json
{"owninguser@odata.bind": "/systemuser(fb2d8cfb-2b19-4454-9caf-3abc80d9c8a3)
}