code: function customCheck(req, mydevice) {
      var useragent = req.headers['user-agent'];
  
      if (!useragent || useragent === '') {
          if (req.headers['cloudfront-is-mobile-viewer'] === 'true') return 'phone';
          if (req.headers['cloudfront-is-tablet-viewer'] === 'true') return 'tablet';
          if (req.headers['cloudfront-is-desktop-viewer'] === 'true') return 'desktop';
          // No user agent.
          return mydevice.parser.options.emptyUserAgentDeviceType;
      }
  
      return mydevice.type;
  }
docs: should this be on 'device' instead?