[BUGFIX] Convert date objects to string in SaveToDatabase finisher

The SaveToDatabase now converts DateTime(Immutable) objects to strings
to make it possible to save values from "Date" or "DatePicker" form elements
into the database.

Resolves: #85246
Releases: master, 8.7
Change-Id: I1533252d462d08bf7b743884d4e8e5fab12f6423
Reviewed-on: https://review.typo3.org/57205


Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Lidia Demin <typecat@gmx.net>
Reviewed-by: Riny van Tiggelen <info@online-gamer.nl>
Tested-by: Mona Muzaffar <mona.muzaffar@gmx.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>