Ensure buffers released in HttpHeadResponseDecorator

See gh-25908

Author: rstoyanchev

spring-web/src/main/java/org/springframework/http/server/reactive/HttpHeadResponseDecorator.java

@@ -56,7 +56,9 @@ public final Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 					.then();
 		}
 		else {
-			return Flux.from(body).then();
+			return Flux.from(body)
+					.doOnNext(DataBufferUtils::release)
+					.then();
 		}
 	}
 

spring-web/src/test/java/org/springframework/http/server/reactive/HttpHeadResponseDecoratorTests.java

@@ -66,8 +66,9 @@ public void writeWithGivenContentLength() {
 
 	@Test // gh-25908
 	public void writeWithGivenTransferEncoding() {
+		Flux<DataBuffer> body = Flux.just(toDataBuffer("data1"), toDataBuffer("data2"));
 		this.response.getHeaders().add(HttpHeaders.TRANSFER_ENCODING, "chunked");
-		this.response.writeWith(Flux.empty()).block();
+		this.response.writeWith(body).block();
 		assertThat(this.response.getHeaders().getContentLength()).isEqualTo(-1);
 	}