Fix XLSX vertical alignment being set to top when only horizontal alignment is set

I am using an existing XLSX file as a template. On some cells, only the horizontal alignment is set to something other than the default. When this is the case, an "alignment" node exists in the XML. The CT_CellAlignment class is currently setting the vertical alignment to top when a "vertical" attribute does not even exist on the "alignment" node. The default vertical value in Excel is bottom align, so most of my cell alignment is being overridden with top when I write to the file.