Try with following statement:

request.setRequestBody("{\"short_description\":\"" + current.short_description + "\",\"caller_id\":\""  + current.caller_id + "\",\"priority\":\"" + current.priority + "\",\"category\":\"" + current.category + "\"}");

If error persists, try changing current.short_description, current.caller_id, current.priority, and current.category with  a static value.