修复Java使用POI合并Excel单元格后，边框不显示的问题

使用Apache POI生成Excel文档时，当进行单元格合并操作后，被合并的单元格边框会消失，使用如下方式可以解决。

• 创建方法：

      public void setBorderStyle(int border, CellRangeAddress region, HSSFSheet sheet, HSSFWorkbook wb){
          RegionUtil.setBorderBottom(border, region, sheet, wb);　　//下边框
          RegionUtil.setBorderLeft(border, region, sheet, wb);     //左边框
          RegionUtil.setBorderRight(border, region, sheet, wb);    //右边框
          RegionUtil.setBorderTop(border, region, sheet, wb);      //上边框
      }

• 调用方式：

      //创建wb,sheet的过程略过
      CellRangeAddress region = new CellRangeAddress(0, 0, i, i+1);    //给定要合并的单元格范围
      sheet.addMergedRegion(region);      //这是合并单元格过程
      setBorderStyle(HSSFCellStyle.BORDER_THIN, region, sheet, wb);   //给合并过的单元格加边框