static headersToGenericObject_(headers) {
    const headersObj = {};
    headers.forEach((value, key) => {
      // Since Edge incorrectly return the header with a leading new line
      // character ('\n'), we trim the header here.
      headersObj[key.trim()] = value;
    });
    return headersObj;
  }
@param {!Headers} headers
@return {!Object<string, string>}
@private