code: @Factory
      public static HeaderRequestMatcher requestHeader(final String headerName,
              final Matcher<? super List<String>> pred) {
          return new HeaderRequestMatcher(pred, headerName);
      }
docs: Factory method to create new instance of this matcher.
  @param headerName name of a request header
  @param pred a predicate to be applied on the request header
  @return new instance of this matcher