Fixed incorrect week numbers in DateField (#14437)

Daylight saving time caused problems with the week number calculation
in the DateTimeService.getISOWeekNumber(Date d) method. If the given
date was inside a DST period, there was a rounding error when calculating
the number of days from the beginning of the year. As a result the week
numbers were calculated incorrectly in a case where the year started with
a Thursday (like for example 2015 and 2026).

Change-Id: Ib3d045ea0b9a1acc44b6f28487b064b3c2b76bb9