static _isRangeRequest (request) {
    const header = request.headers.get('Range');
    if (!header) {
      return false;
    }

    const rangeHeader = header.trim().toLowerCase();
    // Not a range request
    if (!rangeHeader) {
      return false;
    }

    return true;
  }
Thanks to Jake Archibald for the code that became this helper.
@see https://github.com/jakearchibald/range-request-test/blob/master/static/sw.js