XmlDumper doesn't write factory-class attribute.

Symfony\Component\DependencyInjection\Dumper\XmlDumper didn't write 'factory-class' XML attribute for definitions on which setFactoryClass() was set.

This caused the Container[Builder] to throw an exception when the relevant service is being requested/initiated after loading the dumped XML:

`Uncaught Exception Symfony\Component\DependencyInjection\Exception\RuntimeException: "Cannot create service "xxx" from factory method without a factory service or factory class." at /<path>/<to>/vendor/symfony/dependency-injection/Symfony/Component/DependencyInjection/ContainerBuilder.php`

Fixed the problem, and updated relevant test fixture.