Treat empty Accept header as "*/*"

This commit changes the `accept` RequestPredicate so that it treats an
empty `Accept` header as having the value `*/*`, as per the HTTP spec.

Author: poutsma

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

@@ -18,6 +18,7 @@
 
 import java.net.URI;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
@@ -35,7 +36,6 @@
 import org.springframework.http.MediaType;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.util.Assert;
-import org.springframework.util.StringUtils;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.server.WebSession;
 import org.springframework.web.util.UriUtils;
@@ -157,7 +157,12 @@ public static RequestPredicate accept(MediaType... mediaTypes) {
 			@Override
 			public boolean test(ServerRequest.Headers headers) {
 				List<MediaType> acceptedMediaTypes = headers.accept();
-				MediaType.sortBySpecificityAndQuality(acceptedMediaTypes);
+				if (acceptedMediaTypes.isEmpty()) {
+					acceptedMediaTypes = Collections.singletonList(MediaType.ALL);
+				}
+				else {
+					MediaType.sortBySpecificityAndQuality(acceptedMediaTypes);
+				}
 				boolean match = acceptedMediaTypes.stream()
 						.anyMatch(acceptedMediaType -> mediaTypeSet.stream()
 								.anyMatch(acceptedMediaType::isCompatibleWith));

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

@@ -140,7 +140,7 @@ public void accept() throws Exception {
 		MockServerRequest request = MockServerRequest.builder().header("Accept", json.toString()).build();
 		assertTrue(predicate.test(request));
 
-		request = MockServerRequest.builder().build();
+		request = MockServerRequest.builder().header("Accept", MediaType.TEXT_XML_VALUE).build();
 		assertFalse(predicate.test(request));
 	}