add '_route' key to RedirectableUrlMatcher::redirect

When there is a logger, the Symfony\Compoment\HttpKernel\EventListener\RouterListener class expects the '_route' key to exist in order to log the matched route and issues a PHP Notice

    <?php
    date_default_timezone_set('America/Los_Angeles');
    
    require_once __DIR__ . '/../vendor/autoload.php';
    Symfony\Component\HttpKernel\Debug\ErrorHandler::register();
    $app = new Silex\Application();
    $app['debug'] = true;
    $app['monolog.logfile'] = 'C:/TEMP/log.log';
    $app->register(new \Silex\Provider\MonologServiceProvider());
    $app->get('/a/', function() {
        return 'done';
    });
    
    $app->run();

*[url]/a* issues the notice before redirecting to */a/*