Add Keep-Alive to keep the subscription WebSocket open

**Summary**
Playground fails with an error after playing with subscriptions.

**Version**
Not applicable

**Description**
In step 11. of the Apollo tutorial for Android (1), playing with the Subscription in the Playground (2) stopped working after some time with this error:
```
{
"error": "Could not connect to websocket endpoint wss://apollo-fullstack-tutorial.herokuapp.com/graphql. Please check if the endpoint url is correct."
}
```

(1) https://www.apollographql.com/docs/android/tutorial/11-subscriptions/
(2) https://apollo-fullstack-tutorial.herokuapp.com/

Let me know if you need further information.

未确定源文件或测试。首先在链接的 Playground 中复现故障，并跟踪 subscription WebSocket 端点及其连接生命周期。完成的标准是：在持续使用 Playground 期间，subscription WebSocket 保持打开，而不是最终报告连接错误。

由索引模型根据 Issue 内容生成。