Fix start/endOf week for first-weekday-is-monday langs

When trying to fix a weekly dataset export bug I ran into the following: `startOf('week')` should set the day by using `lang()._week.dow` instead of `0`, which is only correct for langs that start their week on a sunday.

Hope I could be of help, this is my first ever public Github excursion, kinda exciting actually :)

grtz,
  Jurgen