Facebook.js doesn't understand beta standyby message.

[Yulgana33]

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)

[Yulgana33]

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

[ouadie-lahdioui]

Hi,

Issue already reported here #962 and here #990, it will be fixed by the PR #978