[BUGFIX] EXT:form - class attribute in container elements is not shown

If a layout for FORM or FIELDSET is definded and it contains a
class attribute, the class attribute is not set in the template.

Resolves: #70070
Releases: master
Change-Id: Ib527cd269383932306805256415e273a76339cb6
Reviewed-on: http://review.typo3.org/43520


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>