Task: Use dates instead of datetimes when setting fields in ActionSucceeded subscribers.

`expectedStartOfWork` and `date_of_completion` are `Date` fields, not `DateTime`. So are the corresponding columns in SQL. This change therefore avoids the SQL DB having to imlicitely convert a datetime to a date and warning about it with

```
Warning: Incorrect date value: '2014-09-11 10:31:57' for column 'completed' at row 1
```
when trying to [sync](https://github.com/4teamwork/opengever.core/blob/master/opengever/globalindex/model/task.py#L120) OGDS tasks <-> Plone Tasks.

I also adapted the tests that incorrectly assumed `datetime` objects.

*(Deliberately did not create a changelog entry, since this was supposed to work this way from the beginning).*

@deiferni @phgross