code: function userInit(payload, callback)
  {
    var type = messaging.getType(payload);
  
    if (payload.sender && payload[type])
    {
      // detach new object from the source
      payload[type].user = clone(payload.sender);
  
      // add user tailored send functions
      // but keep it outside of own properties
      payload[type].user.__proto__ = {send: send.factory(this, payload[type].user)};
    }
  
    callback(null, payload);
  }
docs: Creates `user` object within `message`,
  based on `sender` property.
  
  @this  Fbbot#
  @param {object} payload - messaging envelop object
  @param {function} callback - invoked after type casting is done