code: function(req, res, next) {
  
        var signature = req.headers[MANDRILL_SIGNATURE_HEADER];
        var fullUrl = options.domain + req.url;
        var isValid = validator(fullUrl, req.body, signature);
        var respondWith = responder(res);
  
        if (!signature) {
          respondWith(401, NOT_AUTHORIZED);
        }else if (req.body && req.body.mandrill_events === '[]' && isValid('test-webhook')) {
          respondWith(200, OK);
        }else if (!isValid(options.webhookAuthKey)) {
          respondWith(403, FORBIDDEN);
        } else {
          next();
        }
      }
docs: Express middleware compatible function to process requests and only continure if the signature is valid. Will return 200 if the request is a test request. It does not update the request.
  @param { Request } req
  @param { Response } res
  @param { function } next