headers attribute

Allow to set headers of the callback request.

Example

{
  "request": {
    "method": "GET"
  },
  "response": {
    "status": "OK"
  },
  "callback": {
    "method": "POST",
    "headers": {        
        "X-Foo": "Bar"
    },
    "body": {
        "message": "This request has headers!"
    }
  }
}