week n of some month should be a Sunday

[welch]

"week 2 of last month" should be a Sunday, but it is instead the same as the weekday of the first day of that month.

Details:
momentjs knows that weeks start on Sunday. So our grammar constructs that snap to weeks via a moment's "startOf()" method -- eg, "this week", "last week", "week of" -- properly return a Sunday. But something like "week 2 of this month" simply adds a week to the first day of this month (not a Sunday), when it should also round that moment down to an even week start.

Needs a small change to OrdinalOffset in CalendarOffset in the grammar.