code: public RefundQueryResponse queryByOutTradeNo(String outTradeNo){
          Map<String, String> queryParams = buildQueryParams(WepayField.OUT_TRADE_NO, outTradeNo);
          Map<String, Object> respData = doPost(QUERY, queryParams);
          return renderQueryResp(respData);
      }
docs: 通过商户订单号查询退款
  @param outTradeNo 商户订单号
  @return 退款查询对象，或抛WepayException