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

This prevents the body from getting a 404 class by forcing the query to be non-404 (making every plugin or script know that the current page is not a 404).
Merge branch 'force-non-404' of github.com:mgmartel/timber into mgmartel-force-non-404

* 'force-non-404' of github.com:mgmartel/timber:
  Force the query to not have is_404 true if force_header is other than 404.
Added braces for debug friendliness