fpspreadsheet: Fix invalid xml characters issue in writing xlsx files (see http://forum.lazarus.freepascal.org/index.php/topic,25110.msg152030.html#msg152030).

git-svn-id: http://svn.code.sf.net/p/lazarus-ccr/svn@3300 8e941d3f-bd1b-0410-a28a-d453659cc2b4