Fix handling calculation of duration of all-day recurring task, fixes #840 (#843)

Recurring tasks may specify a dtstart and due date (instead of a duration). If the task was an all-day task, we may have tried to add a duration derived from the difference between start and due.
The result was a non-all-day duration (although in most cases being a multiple of 24h) and adding this to an all-day date resulted in an error. It's fixed by converting the duration into a proper
all-day duration.