Facebook.js doesn't understand beta standyby message.

[Stubbs]

On initialising a new conversation with botkit I get the following error:

TypeError: Cannot read property 'length' of undefined
    at Object.Facebookbot.facebook_botkit.handleWebhookPayload (/app/node_modules/botkit/lib/Facebook.js:266:59)
at /app/components/routes/incoming_webhooks.js:18:20
    at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5)
    at next (/app/node_modules/express/lib/router/route.js:137:13)
    at Route.dispatch (/app/node_modules/express/lib/router/route.js:112:3)
    at Layer.handle [as handle_request] (/app/node_modules/express/lib/router/layer.js:95:5)
    at /app/node_modules/express/lib/router/index.js:281:22
    at Function.process_params (/app/node_modules/express/lib/router/index.js:335:12)
    at next (/app/node_modules/express/lib/router/index.js:275:10)
    at serveStatic (/app/node_modules/serve-static/index.js:75:16)

It's caused by a message arriving at the webhook that doesn't have a message element as part of the payload:

{
   'id': '267342063756729',
   'time': 1503131104641,
   'standby': [ [Object] ] 
}

The payload is actually from a beta feature and the workaround for now is to unsubscribe from them on your pages messenger settings:

[ouadie-lahdioui]

Thanks @Stubbs for the report, PR #978 just being created

[peterswimm]

Issue fixed with merge