Fire `changeDate` on <enter> in input box

If the user types a date into the datepicker input (#318), it would make sense for pressing `<enter>` to trigger `changeDate`, especially since it hides the calendar, leaving the user to believe that their input has been taken.

I tried writing a unit test for this, but I couldn't figure out how to test input. I'm using this patch in a project however, and it behaves as desired.