Usage
-----

```php
<?php

use Silex\Application;
use Silex\Provider\SerializerServiceProvider;
use Symfony\Component\HttpFoundation\Response;

$app = new Application();

$app->register(new SerializerServiceProvider);

// only accept content types supported by the serializer via the assert method.
$app->get("/pages/{id}.{_format}", function ($id) use ($app) {
    // assume a page_repository service exists that returns Page objects. The
    // object returned has getters and setters exposing the state.
    $page = $app['page_repository']->find($id);
    $format = $app['request']->getFormat();

    if (!$page instanceof Page) {
        $this->abort("No page found for id: $id");
    }

    return new Response($app['serializer']->serialize($page, $format), 200, array(
        "Content-Type" => $app['request']->getMimeType($format)
    ));
})->assert("_format", "xml|json")
  ->assert("id", "\d+");
```