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
You don't want to make it even more compact with something like:

```php
if ( $url = $this->get_url_by_slug( $wp_query ) )
{
  wp_redirect( esc_url_raw( $url ) );
  exit;
}
```