[SecurityBundle] Fixed some config node to allow to unset them

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

@@ -190,11 +190,13 @@ class Configuration
                 ->end()
             ->end()
             ->arrayNode('anonymous')
+                ->canBeUnset()
                 ->children()
                     ->scalarNode('key')->defaultValue(uniqid())->end()
                 ->end()
             ->end()
             ->arrayNode('switch_user')
+                ->canBeUnset()
                 ->children()
                     ->scalarNode('provider')->end()
                     ->scalarNode('parameter')->defaultValue('_switch_user')->end()