Force non-404 headers as soon as possible

If force_header is set in Timber::load_template and it's not 404, make sure it happens as early as possible. The overrides used to be called at normal priority (10), but plugins that do stuff with 404s may use the same hook at default priority.
Merge pull request #175 from mgmartel/set_force_header_earlier

Force non-404 headers as soon as possible