@Override
        public void onSubscribe(Subscription subscription) {
            upstreamSubscription = subscription;
            if (pendingDemand > 0) {
                upstreamSubscription.request(pendingDemand);
                pendingDemand = 0;
            }
        }
Closes the deferred stream without setting a delegate.

@throws IllegalStateException if the delegate has been set already or
                              if {@link #close()} or {@link #close(Throwable)} was called already.
core/src/main/java/com/linecorp/armeria/common/stream/DeferredStreamMessage.java