Fix optional ContentNegotiationStrategy for Actuator

Since https://jira.spring.io/browse/SPR-16624, the contract for
`ContentNegotiationStrategy` has been refined and should never return an
empty list if it's got no preference for a media type, but it should
rather respond with a `"*/*"` instead.

This commit fixes the `OptionalPathExtensionContentNegotiationStrategy`
defined in the Spring MVC auto-configuration to have that behavior.

Fixes gh-12900

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfiguration.java

@@ -653,7 +653,7 @@ public class WebMvcAutoConfiguration {
 			Object skip = webRequest.getAttribute(SKIP_ATTRIBUTE,
 					RequestAttributes.SCOPE_REQUEST);
 			if (skip != null && Boolean.parseBoolean(skip.toString())) {
-				return Collections.emptyList();
+				return MEDIA_TYPE_ALL_LIST;
 			}
 			return this.delegate.resolveMediaTypes(webRequest);
 		}

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/WebMvcAutoConfigurationTests.java

@@ -55,6 +55,7 @@ import org.springframework.core.io.Resource;
 import org.springframework.format.support.FormattingConversionService;
 import org.springframework.http.CacheControl;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.mock.web.MockHttpServletRequest;
 import org.springframework.test.util.ReflectionTestUtils;
@@ -62,8 +63,11 @@ import org.springframework.util.StringUtils;
 import org.springframework.validation.Validator;
 import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;
 import org.springframework.web.accept.ContentNegotiationManager;
+import org.springframework.web.accept.ContentNegotiationStrategy;
 import org.springframework.web.accept.ParameterContentNegotiationStrategy;
+import org.springframework.web.accept.PathExtensionContentNegotiationStrategy;
 import org.springframework.web.bind.support.ConfigurableWebBindingInitializer;
+import org.springframework.web.context.request.ServletWebRequest;
 import org.springframework.web.filter.HttpPutFormContentFilter;
 import org.springframework.web.servlet.HandlerAdapter;
 import org.springframework.web.servlet.HandlerExceptionResolver;
@@ -822,6 +826,20 @@ public class WebMvcAutoConfigurationTests {
 				});
 	}
 
+	@Test
+	public void contentNegotiationStrategySkipsPathExtension() throws Exception {
+		ContentNegotiationStrategy delegate = mock(ContentNegotiationStrategy.class);
+		ContentNegotiationStrategy strategy = new WebMvcAutoConfiguration
+				.OptionalPathExtensionContentNegotiationStrategy(delegate);
+
+		MockHttpServletRequest request = new MockHttpServletRequest();
+		request.setAttribute(PathExtensionContentNegotiationStrategy.class
+				.getName() + ".SKIP", Boolean.TRUE);
+		ServletWebRequest webRequest = new ServletWebRequest(request);
+		List<MediaType> mediaTypes = strategy.resolveMediaTypes(webRequest);
+		assertThat(mediaTypes).containsOnly(MediaType.ALL);
+	}
+
 	private void assertCacheControl(AssertableWebApplicationContext context) {
 		Map<String, Object> handlerMap = getHandlerMap(
 				context.getBean("resourceHandlerMapping", HandlerMapping.class));