Forced mb_strtoupper-call to UTF-8

As the mb_strtoupper-function relies on the internal encoding when used
without the second parameter and this does not work all the time (e.g.
for me on Windows äöü were not uppercased) the mb_strtoupper-call
wasforced to UTF-8.
Merge pull request #5 from Mercoline/mbStrToUpper

Force UTF-8 in mb_strtoupper-call