_requestListener(req, res) {
    debug('WebHook request URL: %s', req.url);
    debug('WebHook request headers: %j', req.headers);

    if (req.url.indexOf(this.bot.token) !== -1) {
      if (req.method !== 'POST') {
        debug('WebHook request isn\'t a POST');
        res.statusCode = 418; // I'm a teabot!
        res.end();
      } else {
        req
          .pipe(bl(this._parseBody))
          .on('finish', () => res.end('OK'));
      }
    } else if (this._healthRegex.test(req.url)) {
      debug('WebHook health check passed');
      res.statusCode = 200;
      res.end('OK');
    } else {
      debug('WebHook request unauthorized');
      res.statusCode = 401;
      res.end();
    }
  }
Listener for 'request' event on server
@private
@see https://nodejs.org/docs/latest/api/http.html#http_http_createserver_requestlistener
@see https://nodejs.org/docs/latest/api/https.html#https_https_createserver_options_requestlistener