public static HeaderTemplate create(String name, Iterable<String> values) {
    if (name == null || name.isEmpty()) {
      throw new IllegalArgumentException("name is required.");
    }

    if (values == null) {
      throw new IllegalArgumentException("values are required");
    }

    return new HeaderTemplate(name, values, Util.UTF_8);
  }
Template for HTTP Headers. Variables that are unresolved are ignored and Literals are not
encoded.
core/src/main/java/feign/template/HeaderTemplate.java