Refine check for multiple subscribers

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

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultWebClient.java

@@ -447,8 +447,9 @@ class DefaultWebClient implements WebClient {
 		@SuppressWarnings("unchecked")
 		private <T> Mono<T> drainBody(ClientResponse response, Throwable ex) {
 			// Ensure the body is drained, even if the StatusHandler didn't consume it,
-			// but ignore errors in case it did consume it.
-			return (Mono<T>) response.bodyToMono(Void.class).onErrorMap(ex2 -> ex).thenReturn(ex);
+			// but ignore exception, in case the handler did consume.
+			return (Mono<T>) response.bodyToMono(Void.class)
+					.onErrorResume(ex2 -> Mono.empty()).thenReturn(ex);
 		}
 
 		private static Mono<WebClientResponseException> createResponseException(ClientResponse response) {