Fix 'spring.resources.cache.period' for WebMvc

Before this change it got overwritten by forwarding an empty
CacheControl to Spring. Spring itself sets CacheSeconds already
correctly in absence (=null) of a CacheControl.

Also:
* Fixes bug in WebMvcAutoConfigurationTests.cachePeriod which
prevented it to assert anything

See gh-16488
Closes gh-16730

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

@@ -476,6 +476,10 @@ public class ResourceProperties {
 						.staleIfError(duration.getSeconds(), TimeUnit.SECONDS));
 				map.from(this::getSMaxAge).whenNonNull().to((duration) -> control
 						.sMaxAge(duration.getSeconds(), TimeUnit.SECONDS));
+				// check if cacheControl remained untouched
+				if (control.getHeaderValue() == null) {
+					return null;
+				}
 				return control;
 			}
 

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

@@ -76,7 +76,7 @@ public class ResourcePropertiesTests {
 	public void emptyCacheControl() {
 		CacheControl cacheControl = this.properties.getCache().getCachecontrol()
 				.toHttpCacheControl();
-		assertThat(cacheControl.getHeaderValue()).isNull();
+		assertThat(cacheControl).isNull();
 	}
 
 	@Test

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

@@ -803,13 +803,12 @@ public class WebMvcAutoConfigurationTests {
 		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) {
 				assertThat(((ResourceHttpRequestHandler) handler).getCacheSeconds())
-						.isEqualTo(-1);
+						.isEqualTo(5);
 				assertThat(((ResourceHttpRequestHandler) handler).getCacheControl())
-						.isEqualToComparingFieldByField(
-								CacheControl.maxAge(5, TimeUnit.SECONDS));
+						.isNull();
 			}
 		}
 	}