UTC-mode .weekday() is incorrect

When a moment is in UTC-mode, the `.weekday()` call is wrong because it relies on the internal date's local day-of-week. This also affects a UTC-moment's `.startOf('week')`.

JSFiddle: http://jsfiddle.net/arshaw/ERPA8/

The fix is to make it rely on the moment's intelligent calculation of the current day-of-week.