Have ReactiveTypeHandler consider +x-ndjson suffix as streaming

This commit adds `application/*+x-ndjson`, a wildcard media type which
covers all types that can be parsed as nd-json, to the list of media
types the ReactiveTypeHandler considers for a streaming response in
WebMVC.

As a result, a request which for example `Accept` the
`application/vnd.myapp.v1+x-ndjson` media type will generate a response
with the same `Content-Type`, with newline-delimited json objects being
streamed in the response body.

Closes gh-26817

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

@@ -80,7 +80,8 @@ class ReactiveTypeHandler {
 
 	@SuppressWarnings("deprecation")
 	private static final List<MediaType> JSON_STREAMING_MEDIA_TYPES =
-			Arrays.asList(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_STREAM_JSON);
+			Arrays.asList(MediaType.APPLICATION_NDJSON, MediaType.APPLICATION_STREAM_JSON,
+					MediaType.valueOf("application/*+x-ndjson"));
 
 	private static final boolean isContextPropagationPresent = ClassUtils.isPresent(
 			"io.micrometer.context.ContextSnapshot", ReactiveTypeHandler.class.getClassLoader());
@@ -165,7 +166,7 @@ public ResponseBodyEmitter handleValue(Object returnValue, MethodParameter retur
 				for (MediaType streamingType : JSON_STREAMING_MEDIA_TYPES) {
 					if (streamingType.includes(type)) {
 						logExecutorWarning(returnType);
-						ResponseBodyEmitter emitter = getEmitter(streamingType);
+						ResponseBodyEmitter emitter = getEmitter(type);
 						new JsonEmitterSubscriber(emitter, this.taskExecutor).connect(adapter, returnValue);
 						return emitter;
 					}

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ReactiveTypeHandlerTests.java

@@ -255,6 +255,32 @@ public void writeStreamJson() throws Exception {
 		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
 	}
 
+	@Test
+	public void writeStreamJsonWithVendorSubtype() throws Exception {
+		this.servletRequest.addHeader("Accept", "application/vnd.myapp.v1+x-ndjson");
+
+		Sinks.Many<Bar> sink = Sinks.many().unicast().onBackpressureBuffer();
+		ResponseBodyEmitter emitter = handleValue(sink.asFlux(), Flux.class, forClass(Bar.class));
+
+		assertThat(emitter).as("emitter").isNotNull();
+
+		EmitterHandler emitterHandler = new EmitterHandler();
+		emitter.initialize(emitterHandler);
+
+		ServletServerHttpResponse message = new ServletServerHttpResponse(this.servletResponse);
+		emitter.extendResponse(message);
+
+		Bar bar1 = new Bar("foo");
+		Bar bar2 = new Bar("bar");
+
+		sink.tryEmitNext(bar1);
+		sink.tryEmitNext(bar2);
+		sink.tryEmitComplete();
+
+		assertThat(message.getHeaders().getContentType().toString()).isEqualTo("application/vnd.myapp.v1+x-ndjson");
+		assertThat(emitterHandler.getValues()).isEqualTo(Arrays.asList(bar1, "\n", bar2, "\n"));
+	}
+
 	@Test
 	public void writeText() throws Exception {