try looking up a post by slug if we got a 404

an attempt to solve the issue where permalinks that include dates sometimes do not match up between servers in different timezones.

see https://github.com/GigaOM/gigaom/issues/5121
Good call on the `post_status`. 

I'm also wondering if we should set the `post_type`? If one is set in the `$wp_query` we could use that, and if not perhaps just default to `post`?