Add generics to AbstractCacheManager#caches

Facilitates type-safe programmatic configuration from @Bean methods:

    @Bean
    public CacheManager cacheManager() {
        SimpleCacheManager cm = new SimpleCacheManager();
        cm.setCaches(Arrays.asList(
            new ConcurrentMapCache("default"),
            new ConcurrentMapCache("primary"),
            new ConcurrentMapCache("secondary")
        ));
        return cm;
    }

Prior to this change, the code above would have raised errors on the
Arrays.asList() call because it returns a Collection<? extends Cache>
as opposed to Collection<Cache>.

After this change, AbstractCacheManager expects
Collection<? extends Cache> throughout.

org.springframework.context/src/main/java/org/springframework/cache/support/AbstractCacheManager.java

@@ -44,7 +44,7 @@ public abstract class AbstractCacheManager implements CacheManager, Initializing
 
 
 	public void afterPropertiesSet() {
-		Collection<Cache> caches = loadCaches();
+		Collection<? extends Cache> caches = loadCaches();
 		Assert.notEmpty(caches, "loadCaches must not return an empty Collection");
 		this.cacheMap.clear();
 
@@ -73,6 +73,6 @@ public abstract class AbstractCacheManager implements CacheManager, Initializing
 	 * Load the caches for this cache manager. Occurs at startup.
 	 * The returned collection must not be null.
 	 */
-	protected abstract Collection<Cache> loadCaches();
+	protected abstract Collection<? extends Cache> loadCaches();
 
 }

org.springframework.context/src/main/java/org/springframework/cache/support/SimpleCacheManager.java

@@ -29,17 +29,17 @@ import org.springframework.cache.Cache;
  */
 public class SimpleCacheManager extends AbstractCacheManager {
 
-	private Collection<Cache> caches;
+	private Collection<? extends Cache> caches;
 
 	/**
 	 * Specify the collection of Cache instances to use for this CacheManager.
 	 */
-	public void setCaches(Collection<Cache> caches) {
+	public void setCaches(Collection<? extends Cache> caches) {
 		this.caches = caches;
 	}
 
 	@Override
-	protected Collection<Cache> loadCaches() {
+	protected Collection<? extends Cache> loadCaches() {
 		return this.caches;
 	}