[BUGFIX] EXT:form - subjectField does not work in mail postProcessor

Respect the subjectField configuration property in the
mail postProcessor.

Resolves: #70076
Releases: master
Change-Id: I6534f04745d0c88d38beb7ec1a08eb209d0f77be
Reviewed-on: http://review.typo3.org/43696


Reviewed-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Tested-by: Bjoern Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>