ConvertToSingle - User culture bug fix

This is to fix problems reported to forum
     http://forum.xbmc.org/showthread.php?tid=172394&pid=1585236#pid1585236

PLEASE: Dan, if you have your Windows machine set to de-DE, please run the the unit test for clsAPINumUtils
   * From the Visual Studio menu, select Test, Windows, Test Explorer
   * Click the drop-down to the LEFT of the test search box and select view by CLASS
   * Right-click Test_clsAPINumUtils and select Run Selected Tests

This should compile and run the specific tests under your de-DE culture machine, and hopefully the result will be all-green (passed).

There is no workable way to test as if using a different culture, short of re-installing Windows :( So any help is appreciated.