Fix DX effective date test for Plone 4.3.4.

This fixes the [current test failure](https://jenkins.4teamwork.ch/job/ftw.builder-master-test-plone-4.3.x.cfg/537/console) for Plone 4.3.4.1.

`obj.effective` now seems to be not callable any more (again), and is a `datetime.datetime` instead of `DateTime`. Didn't look into it any further, but this fixes the test for the current Plone version.

@jone @phgross