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

**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()`.