Ensure headers committed immediately for SseEmitter

Issue: SPR-13341

Author: rstoyanchev

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitterReturnValueHandler.java

@@ -121,6 +121,7 @@ public void handleReturnValue(Object returnValue, MethodParameter returnType,
 
 		// Commit the response and wrap to ignore further header changes
 		outputMessage.getBody();
+		outputMessage.flush();
 		outputMessage = new StreamingServletServerHttpResponse(outputMessage);
 
 		DeferredResult<?> deferredResult = new DeferredResult<Object>(emitter.getTimeout());