Fix ignored use-last-modified property in Spring MVC

Prior to this commit, the `spring.web.resources.cache.use-last-modified`
configuration property support was lost during a merge operation right
before 2.4.2.

This is due to an invalid test that missed this unintended change. This
commit fixes both problems.

Fixes gh-26212

Author: bclozel

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

@@ -426,6 +426,7 @@ private void addResourceHandler(ResourceHandlerRegistry registry, String pattern
 			customizer.accept(registration);
 			registration.setCachePeriod(getSeconds(this.resourceProperties.getCache().getPeriod()));
 			registration.setCacheControl(this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl());
+			registration.setUseLastModified(this.resourceProperties.getCache().isUseLastModified());
 			customizeResourceHandlerRegistration(registration);
 		}
 

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

@@ -1009,7 +1009,7 @@ private void assertResourceHttpRequestHandler(AssertableWebApplicationContext co
 			Consumer<ResourceHttpRequestHandler> handlerConsumer) {
 		Map<String, Object> handlerMap = getHandlerMap(context.getBean("resourceHandlerMapping", HandlerMapping.class));
 		assertThat(handlerMap).hasSize(2);
-		for (Object handler : handlerMap.keySet()) {
+		for (Object handler : handlerMap.values()) {
 			if (handler instanceof ResourceHttpRequestHandler) {
 				handlerConsumer.accept((ResourceHttpRequestHandler) handler);
 			}