code: public HttpHeaders addWarning(String warning) {
      if (warning == null) {
        return this;
      }
      if (this.warning == null) {
        this.warning = getAsList(warning);
      } else {
        this.warning.add(warning);
      }
      return this;
    }
docs: Adds the {@code "Warning"} header or {@code null} for none.
  
  <p>Overriding is only supported for the purpose of calling the super implementation and
  changing the return type, but nothing else.
  
  @since 1.28