code: public String query(String deviceInfo, String date, BillType type){
          checkNotNullAndEmpty(date, "date");
          checkNotNull(type, "bill type can't be null");
          Map<String, String> downloadParams = buildDownloadParams(deviceInfo, date, type);
          String billData = Http.post(DOWNLOAD).body(Maps.toXml(downloadParams)).request();
          if (billData.startsWith("<xml>")){
              XmlReaders readers = XmlReaders.create(billData);
              throw new WepayException(
                      readers.getNodeStr(WepayField.RETURN_CODE),
                      readers.getNodeStr(WepayField.RETURN_MSG));
          }
          return billData;
      }
docs: 查询账单
  @param deviceInfo 微信支付分配的终端设备号，填写此字段，只下载该设备号的对账单
  @param date 账单的日期
  @param type 账单类型
  @see me.hao0.wepay.model.enums.BillType
  @return 账单数据