[BUGFIX] Handle utf8 strings in TitleCaseFilter

Use proper charset aware string conversion API to handle
the value given to the TitleCaseFilter.

Resolves: #62279
Releases: master, 6.2
Change-Id: I500af9f1fb6769ea35197e19b2a0a7876d7eb736
Reviewed-on: http://review.typo3.org/33340
Reviewed-by: Philipp Gampe <philipp.gampe@typo3.org>
Tested-by: Philipp Gampe <philipp.gampe@typo3.org>