Add updateDisabledDates and matching test

[happydude]

I created a test to help track down an error I am seeing when changing the disabledDates array

[btecu]

Looks like updateDisabledDates is missing. It should be something like:

  updateDisabledDates() {
    this.picker().disabledDates(this.get('disabledDates'));
  },