When forum or course is hidden, RSS feeds from the forum are not being generated.

This is caused by wrong capability check in rss/file.php:
//Check for "security" if the course is hidden or the activity is hidden
if (Unable to render embedded object: File ($isblog and () not found.$course->visible || !$cm->visible) && (!has_capability('moodle/course:viewhiddenactivities', $context))) {

User ID should be passed to has_capability because current user ($USER) is overwritten at the beginning of the rss/file.php file.

Patch attached.