Add basePath to Router Fixes #1440

[JoeBengalen]

After some chatting with @akrabat about #1440 this is the result.
This should now work in combination with a base path:

$app->get('/books', function () {})->setName('books');

$app->get('/test', function ($request, $response, $args) {
    $uri = $request->getUri()->withPath($this->router->pathFor('books'));

    return $response->withRedirect($uri, 307);
});