code: public static HeaderTemplate append(HeaderTemplate headerTemplate, Iterable<String> values) {
      Set<String> headerValues = new LinkedHashSet<>(headerTemplate.getValues());
      headerValues.addAll(StreamSupport.stream(values.spliterator(), false)
          .filter(Util::isNotBlank)
          .collect(Collectors.toSet()));
      return create(headerTemplate.getName(), headerValues);
    }
docs: Append values to a Header Template.
  
  @param headerTemplate to append to.
  @param values to append.
  @return a new Header Template with the values added.