UTC DateTime activity parameters get converted to local time

This seems to be a regression caused by #442.

If I have an activity defined that takes a `DateTime` parameter and pass it `DateTime.UtcNow`, the activity sees a `DateTime` with `DateTimeKind.Local`. This appears to be related to the recent usage of `DateParseHandling.None` based on some manual testing I did.

The workaround is to use strings instead of `DateTime` and parse them manually.