Fall back to all media types if encountering invalid Accept header

A warn log message is printed, and if log level is set to debug, the
exception stacktrace is logged, too.

Closes gh-37455

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

@@ -23,6 +23,7 @@ import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
 import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.annotation.Value;
@@ -31,6 +32,8 @@ import org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoCon
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider;
 import org.springframework.boot.autoconfigure.template.TemplateAvailabilityProviders;
 import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
+import org.springframework.boot.test.system.CapturedOutput;
+import org.springframework.boot.test.system.OutputCaptureExtension;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -54,7 +57,9 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
  * Tests for {@link WelcomePageHandlerMapping}.
  *
  * @author Andy Wilkinson
+ * @author Moritz Halbritter
  */
+@ExtendWith(OutputCaptureExtension.class)
 class WelcomePageHandlerMappingTests {
 
 	private final WebApplicationContextRunner contextRunner = new WebApplicationContextRunner()
@@ -164,6 +169,17 @@ class WelcomePageHandlerMappingTests {
 			});
 	}
 
+	@Test
+	void logsInvalidAcceptHeader(CapturedOutput output) {
+		this.contextRunner.withUserConfiguration(TemplateConfiguration.class).run((context) -> {
+			MockMvc mockMvc = MockMvcBuilders.webAppContextSetup(context).build();
+			mockMvc.perform(get("/").accept("*/*q=0.8"))
+				.andExpect(status().isOk())
+				.andExpect(content().string("index template"));
+		});
+		assertThat(output).contains("Received invalid Accept header. Assuming all media types are accepted");
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	static class HandlerMappingConfiguration {