[Routing] support for array values in route defaults

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

As pointed out in symfony/symfony-docs#4017, the ``XmlFileLoader`` was not capable of defining array default values.

- [x] array values
- [x] integer values
- [x] float values
- [x] boolean
Please remove the nil test from https://github.com/xabbuh/symfony/blob/route-collection-defaults-xml/src/Symfony/Component/Routing/Tests/Fixtures/validpattern.xml#L16 and put it in your scalar_defaults test.