Allow @Cacheable method to return Optional

This commit further refines 240f254 to also support java.util.Optional
for synchronized cache access (i.e. when the `sync` attribute on
`@Cacheable` is set to `true`).

Issue: SPR-14853

spring-context/src/test/java/org/springframework/cache/CacheReproTests.java

@@ -150,6 +150,22 @@ public class CacheReproTests {
 		assertSame(tb2, cache.get("tb1").get());
 	}
 
+	@Test
+	public void spr14853AdaptsToOptionalWithSync() {
+		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Spr14853Config.class);
+		Spr14853Service bean = context.getBean(Spr14853Service.class);
+		Cache cache = context.getBean(CacheManager.class).getCache("itemCache");
+
+		TestBean tb = new TestBean("tb1");
+		bean.insertItem(tb);
+		assertSame(tb, bean.findById("tb1").get());
+		assertSame(tb, cache.get("tb1").get());
+
+		cache.clear();
+		TestBean tb2 = bean.findById("tb1").get();
+		assertNotSame(tb, tb2);
+		assertSame(tb2, cache.get("tb1").get());
+	}
 
 	@Configuration
 	@EnableCaching
@@ -342,4 +358,33 @@ public class CacheReproTests {
 		}
 	}
 
+	public static class Spr14853Service {
+
+		@Cacheable(value = "itemCache", sync = true)
+		public Optional<TestBean> findById(String id) {
+			return Optional.of(new TestBean(id));
+		}
+
+		@CachePut(cacheNames = "itemCache", key = "#item.name")
+		public TestBean insertItem(TestBean item) {
+			return item;
+		}
+
+	}
+
+	@Configuration
+	@EnableCaching
+	public static class Spr14853Config {
+
+		@Bean
+		public CacheManager cacheManager() {
+			return new ConcurrentMapCacheManager();
+		}
+
+		@Bean
+		public Spr14853Service service() {
+			return new Spr14853Service();
+		}
+	}
+
 }