[BUGFIX] Fix clientside-validation for checkboxes

For all fields val() is used for validation. The val() of a
checkbox is however always set. Instead the checked-state needs
to be taken into account as well.

Change-Id: Ia9b1b892041678c0b4f82c20874bd1d8fde818f3
Resolves: #61136
Reviewed-on: http://review.typo3.org/32312
Reviewed-by: Alex Kellner <alexander.kellner@einpraegsam.net>
Tested-by: Alex Kellner <alexander.kellner@einpraegsam.net>