Even while this is just a demo for module development, I though I submitted this issue:
In file download/views/mod.download.php, line 163:
if ( ! in_array('all', $allowed) && ! in_array($group_id, $allowed))
should be
if ( ! in_array('0', $allowed) && ! in_array($group_id, $allowed))
since the "All" membership is defined in the database table as '0' and not as 'all'. Either that or modifying the part where the database is filled with new entries/ edited entries.
Keep up the great work!