Get a copy of the CT_Border when about to mutate it

[jonopare]

GetCTBorder() returns a shared instance when applyBorder is true. When GetCTBorder() is invoked from inside a property setter, that shared instance is then mutated and all other XSSFCellStyle instances that referred to the original (before mutation) CT_Border are changed as well. This appears to be unintentional. If we know we are inside a setter (and therefore about to mutate the state of the CT_Border) then we should perform that mutation on a copy.

[jonopare]

This change resolves #288 and #499

[jonopare]

This bug can be demonstrated with the following code snippet:

var workbook = new XSSFWorkbook();

var sheet = workbook.CreateSheet("Bug");

var row = sheet.CreateRow(0);

XSSFCell cell;
XSSFCellStyle cellStyle;

cell = (XSSFCell)row.CreateCell(0);
cellStyle = (XSSFCellStyle)workbook.CreateCellStyle();
cellStyle.BorderLeft = BorderStyle.Thin;
cell.CellStyle = cellStyle;
cell.SetCellValue("left:thin");

cell = (XSSFCell)row.CreateCell(1);
cellStyle = (XSSFCellStyle)workbook.CreateCellStyle();
cellStyle.BorderLeft = BorderStyle.Thin;
cellStyle.BorderBottom = BorderStyle.Double;
cell.CellStyle = cellStyle;
cell.SetCellValue("left:thin;bottom:double");

cell = (XSSFCell)row.CreateCell(2);
cellStyle = (XSSFCellStyle)workbook.CreateCellStyle();
cellStyle.BorderLeft = BorderStyle.Thin;
cellStyle.BorderBottom = BorderStyle.Double;
cellStyle.SetBorderColor(BorderSide.LEFT, new XSSFColor(IndexedColors.Red));
cell.CellStyle = cellStyle;
cell.SetCellValue("left:thin+red;bottom:double");

using (var stream = File.Create($"out-{DateTime.Now:yyyyMMddHHmmss}.xlsx"))
{
	workbook.Write(stream);
}

[tonyqus]

Your fix works in this case. But I worry that this change may cause huge number of unnecessary CTBorder clone for any sub-properties settings on border.