Strange situation after closing subscription connection from playground

### What happened?

"ka" frame sending from server to client (graphql web playground) after closing connection ("stop" frame).

### What did you expect?

Connection fully closed, "stop" frame from client and "complete" frame from server are last.

### Minimal graphql.schema and models to reproduce

> type Subscription {
> recommendations: RecommendationElement!
> }
>
> union RecommendationElement = InfoPopup
>
> type InfoPopup {
> title: String!
> }

### versions
- `gqlgen version`: v0.9.0
- `go version`: go1.13.3 darwin/amd64
- dep or go modules? go modules

Hello!

We have one issue with webSocket subscriptions, which reproduced constantly with GraphIQL (Playground):

1) Playground establish connection with server, first frames are "connection init", "start" and "connection_ack";
2) Then server sends some "ka" frames (we use keep alive duration 10 seconds);
3) Then connection stops, using Playground stop button, client sends "stop" and server answers "complete";
4) After this "ka" frames continue coming to client, and this process stopped only, when Web-Server (we use Ingress in GKE), closed client->server connection after timeout;


Seems, "ka" frames ticker must be stopped after "stop" and "complete" message, because go routine continue work without any profit.

Would you help us to solve this problem? Thanks.