Fix issue with CacheRegistry related to mocks.

When a mock returns null for the init() method, no fatal error should be
emitted.

Cake/Cache/CacheRegistry.php

@@ -83,7 +83,7 @@ protected function _create($class, $alias, $config) {
 
 		if (!$instance->init($config)) {
 			throw new Error\Exception(
-				sprintf('Cache engine %s is not properly configured.', $class)
+				sprintf('Cache engine %s is not properly configured.', get_class($instance))
 			);
 		}