Bugfix 10128 WorkingTime/DestinationTime roundtrip calculation errors

If you calculate WorkingTime(StartDate, StopDate) and then
DestinationTime(StartDate, WorkingTime), you sometimes arrive at a time that is
*later* than the original StopDate, which is nonsense.
Re-calculating the WorkingTime with the DestinationTime may in some cases even show an increase.

http://bugs.otrs.org/show_bug.cgi?id=10128