fpspreadsheet: Fix zip/xml path delimiter issue for ooxml making fps-written xlsx files unreadable by Libre/OpenOffice (see //forum.lazarus.freepascal.org/index.php/topic,25110.0.html)

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3299 8e941d3f-bd1b-0410-a28a-d453659cc2b4