Add Keep-Alive to keep the subscription WebSocket open

[cdongieux]

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.

[martinbonnin]

I believe this is a side effect of the Heroku timeouts: https://devcenter.heroku.com/articles/http-routing#timeouts

I'll move this ticket to the apollo-tutorial repo as it's not related to apollo-android (edit: it's moved 🎉 )