Fix incorrect timezone accounting in ISO8601 parsing when time not specified

This fixes the failure of the JulianDate spec "Construct from an ISO8601 local calendar week date, basic format" when in the Australian Eastern time zone.

The test constructs two dates that should be equal to April 7, 1985 at midnight in the local time zone.  On April 7 in 1985, daylight savings time ended at 2am in the Australian Eastern Time timezone.  Since the constructed time is before this, the UTC offset should be +11 hours.  But the code in `JulianDate` was computing the time zone offset by:

`new Date(Date.UTC(year, month - 1, day)).getTimezoneOffset()`

UTC 1985-04-07 at midnight equals AET 1985-04-07 at 10 am.  This is after the 2am end of daylight savings time, so the call to getTimezoneOffset returns 10 hours instead of 11.  The incorrect offset then results in `fromIso8601('1985W15')` returning a time that is not at midnight local time.

I'm not 100% confident in this fix, but it does make the code consistent with the comment above it, so hopefully that's good evidence that this is correct.