Repeatable to-dos: include next in `things.upcoming()`.

[mikez]

To Reproduce Steps to reproduce the behavior:

1. Create a new to-do which repeats monthly (e.g. repeat every month on the 1st day).
2. Run things.upcoming().
3. The next upcoming event is not included; however, it is visible if you go to "Upcoming" in Things.app.

Expected behavior Include the next upcoming event in things.upcoming().