Fix unit-tests with PHP >= 5.4.30
Since PHP 5.4.30, DateTime are not serialized the same way as previous versions.
See http://php.net/ChangeLog-5.php#5.4.30 & https://bugs.php.net/bug.php?id=67308
Merge pull request #1047 from guillaumelecerf/fix_datetime_unittests

Fix unit-tests with PHP >= 5.4.30