docs: public Map<String, Collection<String>> headers() {
      Map<String, Collection<String>> headerMap = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
      this.headers.forEach((key, headerTemplate) -> {
        List<String> values = new ArrayList<>(headerTemplate.getValues());
  
        /* add the expanded collection, but only if it has values */
        if (!values.isEmpty()) {
          headerMap.put(key, Collections.unmodifiableList(values));
        }
      });
      return Collections.unmodifiableMap(headerMap);
    }