Make date format of cookie "Expires" field more unequivocal (YY-> YYYY)

This fix is an attempt to resolve [JENKINS-9258](https://issues.jenkins-ci.org/browse/JENKINS-9258). It seems like winstone creates Expires-values for cookies that contain a two-digit year field. While this is valid according to [RFC 6265](http://tools.ietf.org/html/rfc6265#section-5.1.1) it is also highly ambiguous. So this change makes the year a four-digit field. This should fix problems with cookies being silently dropped.