ChannelSendOperator does not propagate cancel signal to the server

This is a follow-up to SPR-17506 in which writing with Content-Length: 0 and byte[0] was causing hanging. The issue was fixed in 5.1.3 by using response.setComplete() instead of writing an empty byte array. However the underlying issue of hanging with Content-Length: 0 and byte[0] remains.

After further investigation, there is an issue in ChanelSendOperator in that it currently only cancels the upstream write Publisher. So in a case where a cancel happens after the first signal (onNext, onError, onComplete) is emitted, and therefore after the server write Subscriber is hooked in, in that scenario only the upstream write Publisher is notified to stop sending, but the server write Subscriber is not aware.

In addition to the sample in SPR-17506 there is also a sample in https://github.com/reactor/reactor-netty/issues/518.