Force order by date

I use Simplepie to make an aggregate feed. If one day, one of the items from one of the feeds I use to make the aggregate has a bad date, then the order by date sorting is aborted and the order of my whole feed changes.

I would like simplepie to order by date anyway. To that end I added a 'force' option to enable_order_by_date that skips the missing date checks.

I like this because in this case only one item is out of order instead of every item.