code: private static Iterable<String> append(final Response res,
          final int length) throws IOException {
          final String header = "Content-Length";
          return new RsWithHeader(
              new RsWithoutHeader(res, header),
              header,
              Integer.toString(length)
          ).head();
      }
docs: Appends content length to header from response.
  @param res Response
  @param length Response body content length
  @return Iterable String of header attributes
  @throws IOException if something goes wrong.