[BUGFIX] EXT:Scheduler: Post only fields of selected class on save

When saving a new task, now only the extra fields of the selected class
where submitted via post and no longer all extra fields of all possible
classes.
This prevents saving the wrong value if extra fields of different classes
have the same id.

Resolves: #25805
Releases: master
Change-Id: I4f0ae67694fbc3b284e763e3edd123804d078350
Reviewed-on: https://review.typo3.org/51027


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>