[BUGFIX] Re-enable "default" option in Fluid Case ViewHelper

A bug, introduced with the registerArguments option does
not work properly anymore with the "default" option for the <f:case>
viewhelper.

Casting the argument let's the option be evaluated again.

Resolves: #78638
Releases: master
Change-Id: I5b245378db4fab430fc0670b3a4a77de6c3b863f
Reviewed-on: https://review.typo3.org/50575


Reviewed-by: Claus Due <claus@phpmind.net>
Reviewed-by: Benjamin Kott <info@bk2k.info>
Tested-by: Benjamin Kott <info@bk2k.info>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>