[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
The thing with additional element that are not allowed by the XSD is that they are rejected by the loader. That's because it throws exception if it detects elemens it does not know.