Use $res->header('Content-Type') instead of $res->content_type, since the latter can return an array. Thanks to Tatsuhiko Miyagawa for the patch.
$res->is_success, is_error, and is_redirect previously threw an exception when called with NoNetwork. They just work as 200 succesful request now, when the response is taken back from the cache. Thanks to Tatsuhiko Miyagawa for the patch.