docs: private static StringMap filterParam(StringMap params) {
          final StringMap ret = new StringMap();
          if (params == null) {
              return ret;
          }
          params.forEach(new StringMap.Consumer() {
              @Override
              public void accept(String key, Object value) {
                  if (value == null) {
                      return;
                  }
                  String val = value.toString();
                  if ((key.startsWith("x:") || key.startsWith("x-qn-meta-")) && !val.equals("")) {
                      ret.put(key, val);
                  }
              }
          });
          return ret;
      }