Fix memory leak for ServerSentEventHttpMessageWriter

Issue: SPR-17419

Author: poutsma

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

@@ -32,6 +32,8 @@
 import org.springframework.core.codec.Hints;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
+import org.springframework.core.io.buffer.DataBufferUtils;
+import org.springframework.core.io.buffer.PooledDataBuffer;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpOutputMessage;
 import org.springframework.http.server.reactive.ServerHttpRequest;
@@ -144,7 +146,8 @@ private Flux<Publisher<DataBuffer>> encode(Publisher<?> input, ResolvableType el
 
 			return Flux.concat(encodeText(sb, mediaType, factory),
 					encodeData(data, valueType, mediaType, factory, hints),
-					encodeText("\n", mediaType, factory));
+					encodeText("\n", mediaType, factory))
+					.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 		});
 	}