message: fix bug when using Facebook Checkbox Plugin the message.sender.id is not present. fix
diff: @@ -287,6 +287,11 @@ function Facebookbot(configuration) {
  // handle normal messages from users (text, stickers, files, etc count!)
  facebook_botkit.middleware.normalize.use(function normalizeMessage(bot, message, next) {
  + // in case of Checkbox Plug-in sender.id is not present, instead we should look at optin.user_ref
  + if(!message.sender && message.optin && message.optin.user_ref){
  + message.sender = {id: message.optin.user_ref};
  + }
  +
  // capture the user ID
  message.user = message.sender.id;
  