Set default view content-type with WebFlux fn

With this commit, the default content-type defined by the
view (usually "text/html;charset=UTF-8" defined in AbstractView)
is used if any, when none is defined in the response headers.

Issue: SPR-16247

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultRenderingResponseBuilder.java

@@ -20,6 +20,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
+import java.util.List;
 import java.util.Locale;
 import java.util.Map;
 import java.util.function.Consumer;
@@ -177,7 +178,7 @@ class DefaultRenderingResponseBuilder implements RenderingResponse.Builder {
 		public Mono<Void> writeTo(ServerWebExchange exchange, Context context) {
 			ServerHttpResponse response = exchange.getResponse();
 			writeStatusAndHeaders(response);
-			MediaType contentType = exchange.getResponse().getHeaders().getContentType();
+			MediaType responseContentType = exchange.getResponse().getHeaders().getContentType();
 			Locale locale = LocaleContextHolder.getLocale(exchange.getLocaleContext());
 			Stream<ViewResolver> viewResolverStream = context.viewResolvers().stream();
 
@@ -186,7 +187,11 @@ class DefaultRenderingResponseBuilder implements RenderingResponse.Builder {
 					.next()
 					.switchIfEmpty(Mono.error(new IllegalArgumentException("Could not resolve view with name '" +
 							name() +"'")))
-					.flatMap(view -> view.render(model(), contentType, exchange));
+					.flatMap(view -> {
+						List<MediaType> mediaTypes = view.getSupportedMediaTypes();
+						MediaType contentType = (responseContentType == null && !mediaTypes.isEmpty() ? mediaTypes.get(0) : responseContentType);
+						return view.render(model(), contentType, exchange);
+					});
 		}
 
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/function/server/DefaultRenderingResponseTests.java

@@ -28,14 +28,19 @@ import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
 import org.springframework.http.ResponseCookie;
 import org.springframework.mock.http.server.reactive.test.MockServerHttpRequest;
 import org.springframework.mock.web.test.server.MockServerWebExchange;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.reactive.result.view.AbstractView;
 import org.springframework.web.reactive.result.view.View;
 import org.springframework.web.reactive.result.view.ViewResolver;
+import org.springframework.web.reactive.result.view.ViewResolverSupport;
+import org.springframework.web.server.ServerWebExchange;
 
+import static org.junit.Assert.assertEquals;
 import static org.mockito.Mockito.*;
 
 /**
@@ -143,4 +148,37 @@ public class DefaultRenderingResponseTests {
 				.verify();
 	}
 
+	@Test
+	public void defaultContentType() throws Exception {
+		Mono<RenderingResponse> result = RenderingResponse.create("view").build();
+
+		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("http://localhost"));
+		TestView view = new TestView();
+		ViewResolver viewResolver = mock(ViewResolver.class);
+		when(viewResolver.resolveViewName(any(), any())).thenReturn(Mono.just(view));
+
+		List<ViewResolver> viewResolvers = new ArrayList<>();
+		viewResolvers.add(viewResolver);
+
+		ServerResponse.Context context = mock(ServerResponse.Context.class);
+		when(context.viewResolvers()).thenReturn(viewResolvers);
+
+		StepVerifier.create(result.flatMap(response -> response.writeTo(exchange, context)))
+				.verifyComplete();
+
+		assertEquals(ViewResolverSupport.DEFAULT_CONTENT_TYPE, exchange.getResponse().getHeaders().getContentType());
+	}
+
+
+	private static class TestView extends AbstractView {
+
+		@Override
+		protected Mono<Void> renderInternal(Map<String, Object> renderAttributes,
+				MediaType contentType, ServerWebExchange exchange) {
+
+			return Mono.empty();
+		}
+
+	}
+
 }