gently close the connection before leaving the page

Tests/MQTTnet.TestApp.AspNetCore2/wwwroot/app/app.ts

@@ -13,4 +13,8 @@ client.on('connect', () => {
 client.on('message', (topic, message) => {
     // message is Buffer
     console.log(message.toString());
-})
+});
+
+window.onbeforeunload = () => {
+    client.end();
+};