[BUGFIX] Make alphanum. filter work with umlauts

Changes the regular expression used to validate the string into a
unicode-aware character-class and adds two tests for this.

Change-Id: I8cb83376ec3a029a0729cf950d385518746904b4
Fixes: #35284
Releases: 4.6, 4.7, 6.0
Reviewed-on: http://review.typo3.org/10065
Reviewed-by: Andreas Wolf
Tested-by: Andreas Wolf