Accept header negotiation for the output format.

If several output formats are available, allow clients to select one using the `Content-Type` header.
Maybe a bit cleaner:

```php
$restful_handler = $restful_handler ? $restful_handler : restful_get_restful_handler_for_path();
if ($restful_handler && $formatter_name = $restful_handler->getPluginInfo('formatter')) {
  return restful_get_formatter_handler($formatter_name, $restful_handler);
}
```