fixed stop screen sharing

src/lib/VueJanus.vue

@@ -200,10 +200,14 @@
                 this.screenShareStarted=true;
             },
             screenShareStop(){
-                let srcObject=this.$refs.ownstreamscreen.srcObject;
+                var srcObject=this.$refs.ownstreamscreen.srcObject;
                 for (const track of srcObject.getTracks()) {
                     track.stop();
                 }
+                var unpublish = { request: 'unpublish' };
+                this.janusScreenShareHandle.send({ message: unpublish });
+                this.janusScreenShareHandle.detach();
+
                 this.screenShareStarted=false;
             },
             screenShare () {