Replace `_method` requirement by {g,s}etMethods()

Make the call compatible with future Symfony version, and avoid
E_USER_DEPRECATED as thrown by the current 2.7.0-beta1:

The "_method" requirement is deprecated since version 2.2 and will be
removed in 3.0. Use getMethods() instead. at
…/Symfony/Component/Routing/Route.php#554

The "_method" requirement is deprecated since version 2.2 and will be
removed in 3.0. Use the setMethods() method instead or the "methods"
option in the route definition. at
…/Symfony/Component/Routing/Route.php#662