docs: @Override
      public Response register(String CorpNum, Taxinvoice taxinvoice,
                               String UserID, boolean writeSpecification) throws PopbillException {
          if (taxinvoice == null)
              throw new PopbillException(-99999999, "세금계산서 정보가 입력되지 않았습니다.");
  
          String PostData = toJsonString(taxinvoice);
  
          if (writeSpecification) {
              PostData = "{\"writeSpecification\":true," + PostData.substring(1);
          }
  
          return httppost("/Taxinvoice", CorpNum, PostData, UserID,
                  Response.class);
      }