code: private static String convertFifteenToEighteen(String idNumber) {
          if (15 != idNumber.length()) {
              return idNumber;
          }
          idNumber = idNumber.substring(0, 6) + "19" + idNumber.substring(6, 15);
          idNumber = idNumber + getVerifyCode(idNumber);
          return idNumber;
      }
docs: 将15位身份证转换为18位