Fix problem where Excel files couldn't be opened in Ofccie 2010

I haven't traced the problem, the fix is from one of the comments of http://pear.php.net/bugs/bug.php?id=19284, but for me, this reproducably fixes the issue the Excel 2010 refuses to open the files because they are corrupt
Merge pull request #1 from flack/patch-1

Fix problem where Excel files couldn't be opened in Office 2010