[BUGFIX] Checkbox values not shown in emails

Emails sent by FORM sysext do not show the values of checkboxes, in case
of they were checked.

Change-Id: Ie8df935932abb59d211e963776dcdf09c94618c6
Resolves: #33092
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/7721


Tested-by: Harry Glatz <glatz@analog.de>
Reviewed-by: Markus Sommer <markussom@posteo.de>
Tested-by: Markus Sommer <markussom@posteo.de>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>