docs: public static Set<String> varyFields(Headers responseHeaders) {
      Set<String> result = Collections.emptySet();
      for (int i = 0, size = responseHeaders.size(); i < size; i++) {
        if (!"Vary".equalsIgnoreCase(responseHeaders.name(i))) continue;
  
        String value = responseHeaders.value(i);
        if (result.isEmpty()) {
          result = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
        }
        for (String varyField : value.split(",")) {
          result.add(varyField.trim());
        }
      }
      return result;
    }