Complete User Task via REST API

When I use my embeded activiti and run it as a JUnit test im able to complete user task like this

Map<String,Object> userTaskVariableMap =new HashMap<String,Object>();
userTaskVariableMap.put("visa", "Y");
taskService.complete(task.getId(), userTaskVariableMap, false);

How can I complete task with the variable using REST API ?

I tried to put user variable in PUT json body, but it wont work:
PUT http://localhost:8081/activiti-app/api/enterprise/tasks/1023/action/complete