fix spooler handling

src/Symfony/Bundle/SwiftmailerBundle/DependencyInjection/Configuration.php

@@ -61,6 +61,7 @@ class Configuration
                 ->end()
             ->end()
             ->arrayNode('spool')
+                ->addDefaultsIfNotSet()
                 ->scalarNode('type')->defaultValue('file')->end()
                 ->scalarNode('path')->defaultValue('%kernel.cache_dir%/swiftmailer/pool')->end()
             ->end()