code: @Override
      public NutMap send_coupon(String key, WxPayCoupon wxPayCoupon, File file, String password) {
          String url = payBase + "/mmpaymkttransfers/send_coupon";
          Map<String, Object> params = Lang.obj2map(wxPayCoupon);
          return this.postPay(url, key, params, file, password);
      }
docs: 发送代金卷
  
  @param key
  商户KEY
  @param wxPayCoupon
  代金卷内容
  @param file
  证书文件
  @param password
  证书密码
  @return