docs: function onHeaders(ev, status, newUrl, oldUrl, statusCode, method, referrer, headers, resourceType) {
      if (resourceType !== 'mainFrame') return;
  
      let headerArray = [];
  
      Object.keys(headers).forEach(name => {
        headers[name].forEach(value => {
          headerArray.push({ name, value });
        });
      });
  
      response = {
        url: newUrl,
        method,
        status: statusCode,
        contentType: headers['content-type'][0],
        headers: headerArray
      };
  
      notify([ 'header event', status, newUrl, oldUrl, statusCode, method, referrer, headers, resourceType ]);
    }