[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 idea was to ignore elements from another namespace and not throw an exception for unknown tag. So one could extend the loader and xsd to support custom elements, like in FosRestBundle.
I think this won't work with childNodes because it also returns element from other namespaces within the current node.