Milestone and duration change at the same time

[dbarashev]

Scenario:

1. Create a task and make it a milestone
2. Open task properties dialog, uncheck the milestone checkbox (this converts the task to a regular one) and set some duration>1, e.g. 3 days
3. Click OK to apply the changes

Expected: regular task with 3 days duration
Actual: regular task with 1 day duration shown in the chart (in the task table the duration shown is 3 days though)