docs: private String buildRsaPayString(Map<String, String> payParams) {
          String payString = buildSignString(payParams, "\"");
          String sign = RSA.sign(payString, alipay.appPriKey, alipay.inputCharset);
          try {
              sign = URLEncoder.encode(sign, "UTF-8");
          } catch (UnsupportedEncodingException e) {
              throw new AliPayException("sign encode failed", e);
          }
          payString += ("&sign_type=\"" + SignType.RSA.value() + "\"&sign=\""+ sign +"\"");
          return payString;
      }