code: private static Iterable<String> extend(final Iterable<String> head,
          final String header) {
          if (!RsWithHeader.HEADER.matcher(header).matches()) {
              throw new IllegalArgumentException(
                  String.format(
                      // @checkstyle LineLength (1 line)
                      "header line of HTTP response \"%s\" doesn't match \"%s\" regular expression, but it should, according to RFC 7230",
                      header, RsWithHeader.HEADER
                  )
              );
          }
          return new Concat<>(head, Collections.singleton(header));
      }
docs: Add to head additional header.
  @param head Original head
  @param header Value witch will be added to head
  @return Head with additional header