steps to reproduce

1. export 2014CashSheet.xlsx in ZSS
2. load the exported file

actual result

gridline doesn't appear, even we enable gridline.

Debug info

• Copy2014CashSheet.xlsx is exported from 2014CashSheet.xlsx
• The gridline disappears because those cells have a white right and bottom border specified in style

  <div id="_z_684" class="zscell zshi3 zsw1" zs.t="SCell" style="background-color:#ffffff;border-bottom:solid 1px #ffffff;border-right:solid 1px #ffffff;"><div id="_z_684-cave" ...><div id="_z_684-comment" ...></div></div></div>
  

• A2's fill pattern is NONE in 2014CashSheet.xlsx, but become SOLID after exporting (in Copy2014CashSheet.xlsx). Therefore, zss renders a white (#ffffff) border for A1 because the code below:

  //border depends on next cell's fill color if solid pattern
  if(!hitBottom && nextFillStyle !=null){
      //String bgColor = BookHelper.indexToRGB(_book, style.getFillForegroundColor());
      //ZSS-34 cell background color does not show in excel
      String bgColor = nextFillStyle.getFillPattern() == FillPattern.SOLID ? 
              nextFillStyle.getBackColor().getHtmlColor() : null; //ZSS-857
      if (bgColor != null) {
          hitBottom = appendBorderStyle(sb, "bottom", BorderType.THIN, bgColor);
      } else if (nextFillStyle.getFillPattern() != FillPattern.NONE) { //ZSS-841
          sb.append("border-bottom:none;"); // no grid line either
          hitBottom = true;
      }
  }