If you create a Day and call next() it will not have all fields
calculated. It looks intentional, but is it useful?
Pike v7.9 release 11 running Hilfe v3.5 (Incremental Pike Frontend)
> object d = Calendar.ISO.Day(2013,8,14);
> object e = d->next();
> e->md;
(1) Result: -1000
> e->m;
(2) Result: -1000
> e->make_month();
(3) Result: 0
> e->md;
(4) Result: 15
> e->m;
(5) Result: 8
or to put it in more surprising terms
> object d = Calendar.ISO.Day(2013,8,14)->next();
> d->m;
(1) Result: -1000
> d; // Forces _sprintf to call make_month()
(2) Result: Day(Thu 15 Aug 2013)
> d->m;
(3) Result: 8
