Fix iso week parsing for small years

Apparently Date.UTC works only for years after 1900:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date

To form an iso date, from any given year you should use an iso formatted
string. That string doesn't need 'Z' in the end ...
Merge pull request #1350 from ichernev/bugfix/iso-week-parse-small-years

Fix iso week parsing for small years