Date().add(components:) doesn't work when daylight saving time changes

[jsslai]

I noticed Date().add method returns incorrect date when daylight saving time changes between start and end dates. Test case:

func test_additionDuringDaylightSavingTimeChange() {
        let d_helsinki = Region(tz: TimeZoneName.europeHelsinki, cal: CalendarName.gregorian, loc: LocaleName.finnishFinland)
        Date.setDefaultRegion(d_helsinki)

        let date1 = try! DateInRegion(components: [.year: 2016, .month: 10, .day: 24], fromRegion: d_helsinki).absoluteDate // 2016-10-24 00:00:00 GMT+3
        let date2 = try! DateInRegion(components: [.year: 2016, .month: 10, .day: 31], fromRegion: d_helsinki).absoluteDate // 2016-10-31 00:00:00 GMT+2
        let date3 = date1.add(components: [Calendar.Component.weekOfYear: 1]) // 2016-10-30 23:00:00 GMT+2

        let isDaylightSavingForDate1 = TimeZoneName.europeHelsinki.timeZone.isDaylightSavingTime(for: date1)
        let isDaylightSavingForDate2 = TimeZoneName.europeHelsinki.timeZone.isDaylightSavingTime(for: date2)
        XCTAssert(isDaylightSavingForDate1 != isDaylightSavingForDate2, "Daylight saving should change between dates")

        XCTAssert(date2 == date3, "Dates should be equal when adding a week in date") // Now dates are different by 1 hour
    }

I guess date2 and date3 should be equal and the date should be 2016-10-31 00:00:00 GMT+2.

[jsslai]

See #316