[DependencyInjection] fixed missing 'factory-class' attribute in XmlDumper output

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Symfony\Component\DependencyInjection\Dumper\XmlDumper didn't write 'factory-class' XML attribute for definitions on which setFactoryClass() was called.

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 the relevant test fixture.
Hi @fabpot,
While reading the [contribution guidelines document](http://symfony.com/doc/current/contributing/code/patches.html#step-2-work-on-your-patch "Work on your patch") I missed the part saying:
> if you want to fix a bug, use the __oldest__ but still maintained version of Symfony where the bug happens (like 2.3)

I'll resubmit a new PR for 2.3 and close this one.