Can not extend a value after access it for the first time

I don't know, if this is the correct behavior or not, but this throw an exception (In pimple 1x it was ok) : 

```php
$container['mail'] = function ($c) {
    return new \Zend_Mail();
};
$test = $container['mail'];

$container->extend('mail', function($mail, $c) {
    $mail->setFrom($c['mail.default_from']);

    return $mail;
});
```
The test provided by this PR is a fail test. just for demonstrate the problem.