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

@param headerTemplate to append to.
@param values to append.
@return a new Header Template with the values added.
core/src/main/java/feign/template/HeaderTemplate.java