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.

No source file or test is identified. Start by reproducing the failure in the linked Playground and trace the subscription WebSocket endpoint and its connection lifetime. Done means the subscription WebSocket remains open during continued Playground use instead of eventually reporting a connection error.

Written by the indexing model from the issue text.