code: public static NotModified of(String etag, Object... args) {
          touchPayload().message(etag, args);
          return _INSTANCE;
      }
docs: Returns a static NotModified instance which when calling on
  {@link #etag()} method, will return whatever stored in the
  {@link #payload} thread local.
  <p>
  Before return the static instance, the specified etag will
  be stored into the {@link #payload} thread local
  
  @param etag the etag
  @param args the args used to populate the etag
  @return a static instance