Add some stricter casts to PhutilCalendarAbsoluteDateTime serialization
Summary: Ref T11805. This fixes a future issue with comparing edited values: false and 0 were being considered to be an edit.
Test Plan: See next change.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T11805
Differential Revision: https://secure.phabricator.com/D16785