code: function typeCast(payload, callback)
  {
    var type = find(types, function(t){ return (t in payload); });
  
    if (type)
    {
      payload.type = normalize(type);
    }
  
    callback(null, payload);
  }
docs: Casts message type based on the present fields from the list
  
  @this  Fbbot#
  @param {object} payload - message object
  @param {function} callback - invoked after type casting is done