Add doOnDiscard hook for streaming mode

Potential fix for issue reported at
https://github.com/reactor/reactor-netty/issues/1746

spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.java

@@ -141,16 +141,18 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 		}
 
 		if (isStreamingMediaType(contentType)) {
-			return message.writeAndFlushWith(body.map(buffer -> {
-				Hints.touchDataBuffer(buffer, hints, logger);
-				return Mono.just(buffer).doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
-			}));
+			return message
+					.writeAndFlushWith(body.map(buffer -> {
+						Hints.touchDataBuffer(buffer, hints, logger);
+						return Mono.just(buffer).doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
+					}))
+					.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 		}
 
 		if (logger.isDebugEnabled()) {
 			body = body.doOnNext(buffer -> Hints.touchDataBuffer(buffer, hints, logger));
 		}
-		return message.writeWith(body);
+		return message.writeWith(body).doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 	}
 
 	@Nullable