Add 'itunes:duration' tag for items with duration (#3774)

* [{Atom,Mrss}Format] Allow itunes tags on items without enclosure

* [Arte7Bridge] Add $item['itunes']['duration'] value

bridges/Arte7Bridge.php

@@ -156,6 +156,10 @@ class Arte7Bridge extends BridgeAbstract
             . $element['mainImage']['url']
             . '" /></a>';
 
+            $item['itunes'] = [
+                'duration' => $durationSeconds,
+            ];
+
             $this->items[] = $item;
         }
     }