WebSocket handler in GRAPHQL_TRANSPORT_WS protocol should not send GQL_COMPLETE when closing connection

Web socket handler for recently implemented `graphql-transport-ws` protocol sends invalid message type when closing connection:

https://github.com/graphql-dotnet/graphql-client/blob/master/src/GraphQL.Client/Websocket/GraphQLTransportWSProtocolHandler.cs#L269

which results in message:
```json
{"type": "complete"}
```

According to [graphql-transport-ws](https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md#complete) messages with type `complete` should contain an `id` of a subscription.

I'm using this library with a server written in different language and for every such message I'm getting a response `"Missing map member: 'id'"` - which I believe is justified.

IMO when closing `graphql-transport-ws` connection only active subscriptions should be closed but no additional messages should be sent.