[Serializer] Example from documentation does not work

[javiereguiluz]

Moved from symfony/symfony#24783 created by @enumag

---

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	no
Symfony version	3.3

I was reading the documentation of Symfony/Serializer component. Here I found info about a new feature in Symfony 3.3, the allow_extra_attributes option. However the example from documentation does not work. This should throw a Symfony\Component\Serializer\Exception\ExtraAttributesException but it does not:

<?php declare(strict_types = 1);

require __DIR__ . '/vendor/autoload.php';

(new \Symfony\Component\Dotenv\Dotenv())->load(__DIR__.'/.env');

$kernel = new \App\Kernel('dev', true);
$kernel->boot();

$container = $kernel->getContainer();
$serializer = $container->get('serializer');

class Person
{
	private $age;
	private $name;
	private $sportsman;

	// Getters
	public function getName()
	{
		return $this->name;
	}

	public function getAge()
	{
		return $this->age;
	}

	// Issers
	public function isSportsman()
	{
		return $this->sportsman;
	}

	// Setters
	public function setName($name)
	{
		$this->name = $name;
	}

	public function setAge($age)
	{
		$this->age = $age;
	}

	public function setSportsman($sportsman)
	{
		$this->sportsman = $sportsman;
	}
}

$data = <<<EOF
<person>
    <name>foo</name>
    <age>99</age>
    <city>Paris</city>
</person>
EOF;

$person = $serializer->deserialize($data, 'Person', 'xml', array(
	'allow_extra_attributes' => false,
));

var_export($person);

cc @juliendidier ref #19958, #6975

[javiereguiluz]

Closing because it was a code issue after all.