bug in moment.add()

[vahid18u]

imagine we have a moment m which it's time is 00:00:00 am, and we call m.add(10, 'day') and suddenly the new day is a day with advanced clock for daylight saving. the new time will be 01:00:00 am surprisingly. this means in case we need to have start of the day, we should call m.startof('day') after any call to m.add(10, 'day').