Force the query to not have is_404 true if force_header is other than 404

Hi Jared!

I had this fix sitting around in my local branch for a while. When I just finally got around to pushing I saw that you already fixed this. Nonetheless, this is a slight improvement, as it fixes the root of the problem (WP Query thinking it's a 404) rather than the symptom (the body getting the 404 class).

Cheers,
Mike