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