message: End invalid Slack signature response.
  When the Slack adapter receives an invalid signature header, it sets the status
  code of the response to 401 but does not end the response.
  This causes requests to hang that do not have a valid signature. This commit
  therefore fixes the issue
diff: @@ -475,6 +475,7 @@ export class SlackAdapter extends BotAdapter {
  if (!validSignature()) {
  debug('Signature verification failed, Ignoring message');
  res.status(401);
  + res.end();
  return false;
  }
  }
  