Commit #c187cb2 introduced proactive rejection of multiple subscribers
in ReactorClientHttpResponse, instead of hanging indefinitely as per
https://github.com/reactor/reactor-netty/issues/503.
However FluxReceive also rejects subsequent subscribers if the response
is consumed fully, as opposed to being canceled, e.g. as with
bodyToMono(Void.class). In that case, a subsequent subscriber causes
two competing error signals to be sent, and one gets dropped and
logged by reactor-core.
This fix ensures that a rejection is raised in
ReactorClientHttpResponse only after a cancel() was detected.
Issue: SPR-17564
7a5f8e03bc