Fix a RRULE issue with BYDAY and similar constraints in noninitial months

Summary: Ref T10747. This latest test failed beacuse the start date (the 2nd) carried over to the first month we could actually examine.

Test Plan: Unit test now passes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16608