code: @Override
      public Client modified(Date date, boolean ifNot) {
          SimpleDateFormat dateFormat = HttpUtils.getHttpDateFormat();
          String hName = ifNot ? HttpHeaders.IF_UNMODIFIED_SINCE : HttpHeaders.IF_MODIFIED_SINCE;
          state.getRequestHeaders().putSingle(hName, dateFormat.format(date));
          return this;
      }
docs: {@inheritDoc}