Fix graceful shutdown

[shunyy]

Fixed invalid context value passing to context.WithTimeout for graceful-shutdown.

Current generated example of graceful-shutdown is calling context.WithTimeout with already closed context, because of that server shutdown immediately. I don't think it is intentional behavior, so fixed with passing context.Background to context.WithTimeout

explanation

<-ctx.Done() // Closed!!

ctx, cancel := context.WithTimeout(ctx, 30*time.Second) // already closed ctx 😢 
defer cancel()

srv.Shutdown(ctx) // does not wait for timeout as intended

changes to examples can be reviewed in: goadesign/examples#71

[raphael]

Whoops, thank you so much! great catch.

[raphael]

Would you mind back-porting this fix to v2 please? simply cherry-pick the merge commit - thank you!