Grouping columns works only with a blank sheet

[pragmaware]

In certain cases XSSFSheet.GroupColumn() produces a overly large columns array in the xml output.
LibreOffice seems to interpret it properly (not sure if always) while Office 365 will either group the wrong number of columns or display no outline at all.

I've found out that it seems to work when it's applied to an empty sheet, when columns aren't present yet. When it's applied after some columns have been created then it seems to multiply them internally and produce wrong results.

The multiplication seems to happen in columnHelper.AddCleanColIntoCols(colsArray, newCol). In a debug session I did the following:

1. created a sheet with 34 columns filled with some data and attributes
2. grouped columns from 4 to 31
3. after AddCleanColIntoCols() the column array contained 93 entries (!) and these were reflected in the produced xlsx file.

If I swap steps 1 and 2 then the resulting column array seems to be correct.

[artem-iron]

Tested it with #1261 and at least the described behaviour isn't reproduced.

EDIT: To be fair, even on the master branch this isn't reproduced.

Tried with this code:

public void CheckIssue728()
{
    XSSFWorkbook workbook = new XSSFWorkbook();
    XSSFSheet sheet = (XSSFSheet)workbook.CreateSheet("Sheet 1");
    for (int i = 0; i < 34; i++)
    {
        for (int j = 0; j < 34; j++)
        {
            var row = sheet.GetRow(i) ?? sheet.CreateRow(i);
            var cell = row.CreateCell(j);
            cell.SetCellValue("cell" + i + j);
        }
    }

    sheet.GroupColumn(4, 31);

    using (FileStream fs = new FileStream("test.xlsx", FileMode.Create, FileAccess.ReadWrite))
    {
        workbook.Write(fs);
    }

    workbook.Dispose();
}