docs: function responseCallback(res) {
          // console.log("statusCode: ", res.statusCode, "headers: ",res.headers);
          self.statusCode = res.statusCode;
  
          // extract BF cookie
          var cookies = res.headers['set-cookie'];
          if (cookies) {
              for ( var i in cookies) {
                  var parsed = cookie.parse(cookies[i]);
                  if (parsed[cookieName]) {
                      self.responseCookie = parsed[cookieName];
                      break;
                  }
              }
          }
  
          // just for statistics of compression efficiency
          res.on('data', function(data) {
              self.rawResponseLength += data.length;
          })
  
          // http request input to self output
          if (res.headers['content-encoding'] === 'gzip') {
              // piping through gzip
              var gunzip = zlib.createGunzip();
              res.pipe(gunzip).pipe(self);
          } else {
              // piping directly to self
              res.pipe(self);
          }
      }