Fix an RRULE issue with month days at the end of the month

Summary:
Ref T10747. I'm porting unit tests from other implementations until I hit one that fails.

The "yearly event on Feb 29th" test failed because we assumed every month had a 29th. Instead, test if the current month has a 29th.

We were also using cursorX instead of stateX in a couple places. They often have the same value, but stateX is the correct value when they differ.

Test Plan: Unit tests now pass.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16605