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

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.
bug #11558 [DependencyInjection] Fixed missing 'factory-class' attribute in XmlDumper output (kerdany)

This PR was merged into the 2.3 branch.

Discussion
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[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

Problem: XmlDumper doesn't write 'factory-class' XML attribute for definitions on which setFactoryClass() was called.

Impact: Container[Builder] to throws 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>//DependencyInjection/ContainerBuilder.php`

Solution: Made XmlDumper write the 'factory-class' attribute, and updated the relevant test fixture.

Another related problem, is that XMLFileLoader doesn't complain if the 'factory-class' attribute is missing for a 'service' elements that include 'factory-method' attribute, resulting in an ill-configured Definition object in the ContainerBuilder. I'll post an issue/ticket, and probably send another PR for that.

Commits
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18e3e6f [DependencyInjection] fixed missing 'factory-class' attribute in XmlDumper output