code: private static void addStringListHeader(Request<?> request, String header, List<String> values) {
          if (values != null && !values.isEmpty()) {
              request.addHeader(header, ServiceUtils.join(values));
          }
      }
docs: <p>
  Adds the specified string list header, joined together separated with
  commas, to the specified request.
  This method will not add a string list header if the specified values
  are <code>null</code> or empty.
  </p>
  
  @param request
  The request to add the header to.
  @param header
  The header name.
  @param values
  The list of strings to join together for the header value.