[Security] allow to use `method` in XML configs

Before this change, you always had to use the `methods` key which is
inconsistent compared to other options like `roles` and `ips` for which
it was possible to use their singular versions.

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

@@ -158,6 +158,7 @@ class MainConfiguration implements ConfigurationInterface
                     ->cannotBeOverwritten()
                     ->prototype('array')
                         ->fixXmlConfig('ip')
+                        ->fixXmlConfig('method')
                         ->children()
                             ->scalarNode('requires_channel')->defaultNull()->end()
                             ->scalarNode('path')