Imply text/event-stream when serializing ServerSentEvent

Issue: SPR-14748

spring-web-reactive/src/main/java/org/springframework/web/reactive/config/WebReactiveConfiguration.java

@@ -405,12 +405,12 @@ public class WebReactiveConfiguration implements ApplicationContextAware {
 		if (jaxb2Present) {
 			writers.add(new EncoderHttpMessageWriter<>(new Jaxb2XmlEncoder()));
 		}
+		writers.add(new Jackson2ServerHttpMessageWriter(new ServerSentEventHttpMessageWriter(sseDataEncoders)));
 		if (jackson2Present) {
 			Jackson2JsonEncoder jacksonEncoder = new Jackson2JsonEncoder();
 			writers.add(new Jackson2ServerHttpMessageWriter(new EncoderHttpMessageWriter<>(jacksonEncoder)));
 			sseDataEncoders.add(jacksonEncoder);
 		}
-		writers.add(new Jackson2ServerHttpMessageWriter(new ServerSentEventHttpMessageWriter(sseDataEncoders)));
 	}
 	/**
 	 * Override this to modify the list of message writers after it has been

spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/SseIntegrationTests.java

@@ -120,6 +120,24 @@ public class SseIntegrationTests extends AbstractHttpHandlerIntegrationTests {
 				);
 	}
 
+	@Test
+	public void sseAsEventWithoutAcceptHeader() throws Exception {
+		Flux<String> result = this.webClient
+				.perform(get("http://localhost:" + port + "/sse/event"))
+				.extract(bodyStream(String.class))
+				.filter(s -> !s.equals("\n"))
+				.map(s -> s.replace("\n", ""))
+				.take(2);
+
+		TestSubscriber
+				.subscribe(result)
+				.await(Duration.ofSeconds(5))
+				.assertValues(
+						"id:0:bardata:foo",
+						"id:1:bardata:foo"
+				);
+	}
+
 	@RestController
 	@SuppressWarnings("unused")
 	static class SseController {

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

@@ -63,7 +63,8 @@ public class ServerSentEventHttpMessageWriter implements HttpMessageWriter<Objec
 
 	@Override
 	public boolean canWrite(ResolvableType elementType, MediaType mediaType) {
-		return mediaType == null || TEXT_EVENT_STREAM.isCompatibleWith(mediaType);
+		return mediaType == null || TEXT_EVENT_STREAM.isCompatibleWith(mediaType) ||
+				ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
 	}
 
 	@Override