Make ArrayHelperTest resilient against undefined sort order

The documentation of usort (http://us1.php.net/manual/en/function.usort.php)
makes it clear that "If two members compare as equal, their relative order in the sorted array is undefined."
Archive exception message fix and make the tests run
Merge pull request #314 from elgenie/master

Make ArrayHelperTest resilient against undefined sort order
Merge remote-tracking branch 'joomla/master' into staging
Merge pull request #306 from florianv/archive

Archive exception message fix and make the tests run