[BUGFIX] DateValidator fails to validate incorrect dates

[BUGFIX] DateValidator fails to validate incorrect dates

If the function "strptime" is not present in the environment (mostly on
Windows systems) the fallback to check a given date in the dateValidator
fails to detect invalid dates. The conversion from the given string to
a date object respects invalid dates and just converts them to valid
ones (by moving in next month/year). Therefore the function returns an
object instead of false. This patch adds a value comparison between old
and new value to be sure still the same date string is returned.

Releases: master, 6.2
Resolves: #64661
Change-Id: I739d309278143bb5fc24c48ca7bb884a1e25a442
Reviewed-on: http://review.typo3.org/36469
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Steffen Müller <typo3@t3node.com>
Tested-by: Steffen Müller <typo3@t3node.com>