Remove quality parameter from selected media type

Prior to this commit, WebFlux application would keep the quality
parameter from the "Accept" request header when selecting a media type
for the response. It would then echo it back to the client.

While strictly not wrong, this is unnecessary and can confuse HTTP
clients. This commit aligns WebFlux's behavior with Spring MVC.

Fixes gh-24239

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

@@ -156,6 +156,7 @@ else if (mediaType.isPresentIn(ALL_APPLICATION_MEDIA_TYPES)) {
 		}
 
 		if (selected != null) {
+			selected = selected.removeQualityValue();
 			if (logger.isDebugEnabled()) {
 				logger.debug("Using '" + selected + "' given " + acceptableTypes +
 						" and supported " + producibleTypes);

spring-webflux/src/test/java/org/springframework/web/reactive/result/HandlerResultHandlerTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,7 +51,7 @@ public class HandlerResultHandlerTests {
 
 
 	@Test
-	public void usesContentTypeResolver() throws Exception {
+	void usesContentTypeResolver() {
 		TestResultHandler resultHandler = new TestResultHandler(new FixedContentTypeResolver(IMAGE_GIF));
 		List<MediaType> mediaTypes = Arrays.asList(IMAGE_JPEG, IMAGE_GIF, IMAGE_PNG);
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path"));
@@ -61,7 +61,7 @@ public void usesContentTypeResolver() throws Exception {
 	}
 
 	@Test
-	public void producibleMediaTypesRequestAttribute() throws Exception {
+	void producibleMediaTypesRequestAttribute() {
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path"));
 		exchange.getAttributes().put(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, Collections.singleton(IMAGE_GIF));
 
@@ -72,7 +72,7 @@ public void producibleMediaTypesRequestAttribute() throws Exception {
 	}
 
 	@Test  // SPR-9160
-	public void sortsByQuality() throws Exception {
+	void sortsByQuality() {
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path")
 				.header("Accept", "text/plain; q=0.5, application/json"));
 
@@ -83,7 +83,7 @@ public void sortsByQuality() throws Exception {
 	}
 
 	@Test
-	public void charsetFromAcceptHeader() throws Exception {
+	void charsetFromAcceptHeader() {
 		MediaType text8859 = MediaType.parseMediaType("text/plain;charset=ISO-8859-1");
 		MediaType textUtf8 = MediaType.parseMediaType("text/plain;charset=UTF-8");
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path").accept(text8859));
@@ -93,14 +93,25 @@ public void charsetFromAcceptHeader() throws Exception {
 	}
 
 	@Test // SPR-12894
-	public void noConcreteMediaType() throws Exception {
+	void noConcreteMediaType() {
 		List<MediaType> producible = Collections.singletonList(ALL);
 		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path"));
 		MediaType actual = this.resultHandler.selectMediaType(exchange, () -> producible);
 
 		assertThat(actual).isEqualTo(APPLICATION_OCTET_STREAM);
 	}
 
+	@Test
+	void removeQualityParameter() {
+		MockServerWebExchange exchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path")
+				.header("Accept", "text/plain; q=0.5"));
+
+		List<MediaType> mediaTypes = Arrays.asList(APPLICATION_JSON, TEXT_PLAIN);
+		MediaType actual = this.resultHandler.selectMediaType(exchange, () -> mediaTypes);
+
+		assertThat(actual).isEqualTo(TEXT_PLAIN);
+	}
+
 
 	@SuppressWarnings("WeakerAccess")
 	private static class TestResultHandler extends HandlerResultHandlerSupport {