Fix an RRULE issue where BYMONTHDAY did not properly expand the month set

Summary: Ref T10747. When you have BYMONTHDAY, we need to expand the set of months by default, per the comment.

Test Plan: Unit test now passes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16606