clear mpris metadata on stop

src/mpris.vala

@@ -140,6 +140,10 @@ class MprisPlayer : Object {
             this.metadata = metadata;
         });
 
+        playbin.stopped.connect (() => {
+            this.metadata = new HashTable<string, Variant> (null, null);
+        });
+
         this.on_next.connect (() => playbin.go_to_next_track ());
         this.on_previous.connect (() => playbin.go_to_prev_track ());
         this.on_play.connect (() => playbin.play ());