[BUGFIX] Allow subclasses of \DateTime as entity properties

When a class "\MyCustomDateTime extends \DateTime", the data mapper
creates "\DateTime" objects instead when mapping database results.
This patch uses the class annotation to resolve the class to create.

Change-Id: I45062d4d1a44333016258289ea9e2026d9923f70
Resolves: #66939
Releases: master, 6.2
Reviewed-on: http://review.typo3.org/39534


Reviewed-by: Susanne Moog <typo3@susannemoog.de>
Tested-by: Susanne Moog <typo3@susannemoog.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Andreas Wolf <andreas.wolf@typo3.org>
Tested-by: Andreas Wolf <andreas.wolf@typo3.org>