[Subscription] check already closed channels

[cheina97]

This PR fixes an issue I encountered using subscriptions with genqlient.

I've noticed that the Unsubscribe function closes the subscription channel, but never checks if that channel has already been closed.

So a snippet like that causes a panic when Close is called:

err = gqlsubcl.Unsubscribe(subscriptionID)
if err != nil {
	return fmt.Errorf("failed to unsubscribe from GraphQL subscription: %w", err)
}

if err := gqlsubcl.Close(); err != nil {
	return fmt.Errorf("failed to close GraphQL subscription client: %w", err)
}

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[benjaminjkraft]

Thanks, this looks good now! I forgot to mention earlier, could you add a changelog entry? Once you do I think we're good!

[cheina97]

Thanks, this looks good now! I forgot to mention earlier, could you add a changelog entry? Once you do I think we're good!

Yes, I just did it

[cheina97]

@benjaminjkraft, is it ok?

[benjaminjkraft]

Thanks!