Fixed goroutine leak in subscriptions

[korotin]

We have goroutine leak here if events will happen in following order:

• In our goroutine we've read a new message from responses channel, but didn't send it to c channel yet
• Subscriber got disconnected, which forced context to close
• After context was closed, goroutine in graphql-ws-transport stopped reading c channel (https://github.com/graph-gophers/graphql-transport-ws/blob/master/graphqlws/internal/connection/connection.go#L233-L235)
• Now, back to our goroutine, we're trying to send message to c channel. But since that channel is read by no one now, we stuck here forever.

Described situation may seem rare, but in our project with hundreds events per second that leak happened rather often. We tested this fix in production environment and it solved the problem.

@pavelnikolov

[pavelnikolov]

Thank you for this fix.