WriteResultPublisher does not pass cancel signals

[rstoyanchev]

Typically server side response writing shouldn't see cancellation since ServletHttpHandlerAdapter doesn't do that. However it may occur for other reasons such as a WebFilter with a timeout operator on the outbound side. Even so in that case the timeout operator would also send an error signal downstream, which reaches ServletHttpHandlerAdapter and completes the AsyncContetx and that in turn cancels writing. Nevertheless it is a good idea to support cancellation and to cancel writing immediately.

This was uncovered while investigating #26434.