Take content-type from ResponseEntity unconditionally

From #9a894a we began using the content-type from the ResponseEntity
but cross-checking it still against the requested content type.

Arguably there isn't any purpose in cross-checking. The only possible
outcomes are: a) it's compatible or b) it's not, which would result in
406 (NOT_ACCEPTABLE). As we've been given explicitly the media type to
use, it makes little sense to send 406, ignoring the wish to use that
content type.

Issue: SPR-16251

spring-webflux/src/main/java/org/springframework/web/reactive/result/HandlerResultHandlerSupport.java

@@ -116,6 +116,11 @@ public abstract class HandlerResultHandlerSupport implements Ordered {
 	protected MediaType selectMediaType(ServerWebExchange exchange,
 			Supplier<List<MediaType>> producibleTypesSupplier) {
 
+		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
+		if (contentType != null && contentType.isConcrete()) {
+			return contentType;
+		}
+
 		List<MediaType> acceptableTypes = getAcceptableTypes(exchange);
 		List<MediaType> producibleTypes = getProducibleTypes(exchange, producibleTypesSupplier);
 
@@ -152,10 +157,6 @@ public abstract class HandlerResultHandlerSupport implements Ordered {
 	private List<MediaType> getProducibleTypes(ServerWebExchange exchange,
 			Supplier<List<MediaType>> producibleTypesSupplier) {
 
-		MediaType contentType = exchange.getResponse().getHeaders().getContentType();
-		if (contentType != null && contentType.isConcrete()) {
-			return Collections.singletonList(contentType);
-		}
 		Set<MediaType> mediaTypes = exchange.getAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
 		return (mediaTypes != null ? new ArrayList<>(mediaTypes) : producibleTypesSupplier.get());
 	}

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

@@ -22,7 +22,6 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
-import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
 import java.util.Set;
@@ -210,37 +209,41 @@ public abstract class AbstractMessageConverterMethodProcessor extends AbstractMe
 			}
 		}
 
-		HttpServletRequest request = inputMessage.getServletRequest();
-		List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
 
-		MediaType contentType = outputMessage.getHeaders().getContentType();
-		List<MediaType> producibleMediaTypes = (contentType != null && contentType.isConcrete() ?
-				Collections.singletonList(contentType) : getProducibleMediaTypes(request, valueType, declaredType));
+		List<MediaType> mediaTypesToUse;
 
-		if (outputValue != null && producibleMediaTypes.isEmpty()) {
-			throw new HttpMessageNotWritableException("No converter found for return value of type: " + valueType);
+		MediaType contentType = outputMessage.getHeaders().getContentType();
+		if (contentType != null && contentType.isConcrete()) {
+			mediaTypesToUse = Collections.singletonList(contentType);
 		}
+		else {
+			HttpServletRequest request = inputMessage.getServletRequest();
+			List<MediaType> requestedMediaTypes = getAcceptableMediaTypes(request);
+			List<MediaType> producibleMediaTypes = getProducibleMediaTypes(request, valueType, declaredType);
 
-		Set<MediaType> compatibleMediaTypes = new LinkedHashSet<>();
-		for (MediaType requestedType : requestedMediaTypes) {
-			for (MediaType producibleType : producibleMediaTypes) {
-				if (requestedType.isCompatibleWith(producibleType)) {
-					compatibleMediaTypes.add(getMostSpecificMediaType(requestedType, producibleType));
+			if (outputValue != null && producibleMediaTypes.isEmpty()) {
+				throw new HttpMessageNotWritableException(
+						"No converter found for return value of type: " + valueType);
+			}
+			mediaTypesToUse = new ArrayList<>();
+			for (MediaType requestedType : requestedMediaTypes) {
+				for (MediaType producibleType : producibleMediaTypes) {
+					if (requestedType.isCompatibleWith(producibleType)) {
+						mediaTypesToUse.add(getMostSpecificMediaType(requestedType, producibleType));
+					}
 				}
 			}
-		}
-		if (compatibleMediaTypes.isEmpty()) {
-			if (outputValue != null) {
-				throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
+			if (mediaTypesToUse.isEmpty()) {
+				if (outputValue != null) {
+					throw new HttpMediaTypeNotAcceptableException(producibleMediaTypes);
+				}
+				return;
 			}
-			return;
+			MediaType.sortBySpecificityAndQuality(mediaTypesToUse);
 		}
 
-		List<MediaType> mediaTypes = new ArrayList<>(compatibleMediaTypes);
-		MediaType.sortBySpecificityAndQuality(mediaTypes);
-
 		MediaType selectedMediaType = null;
-		for (MediaType mediaType : mediaTypes) {
+		for (MediaType mediaType : mediaTypesToUse) {
 			if (mediaType.isConcrete()) {
 				selectedMediaType = mediaType;
 				break;