Fixed ApcStorage::isAvailable()

That `apc` extension is loaded may not mean it is enabled (with `apc.enable` for webserver or `apc.enable_cli` for CLI).

Nette/Caching/ApcStorage.php

@@ -46,7 +46,7 @@ class ApcStorage implements ICacheStorage
 	 */
 	public static function isAvailable()
 	{
-		return extension_loaded('apc');
+		return extension_loaded('apc') && @apc_cache_info() !== FALSE;
 	}
 
 
@@ -187,4 +187,4 @@ private function _normalizeKey(&$key)
 		$key = $this->prefix . str_replace("\x00", '~', $key); // due http://pecl.php.net/bugs/bug.php?id=17597
 	}
 
-}
+}