docs: @Override
      public SentMessage[] getMessagesRN(String CorpNum, String requestNum)
              throws PopbillException {
          if (requestNum == null)
              throw new PopbillException(-99999999, "전송요청번호가 입력되지 않았습니다.");
  
          return httpget("/Message/Get/" + requestNum, CorpNum, null,
                  SentMessage[].class);
      }