[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
It should. Though I don't think that the XPath is necessary at all. I changed that to use the ``childNodes`` attributes like it is done in the map part. The schema doesn't allow to add other elements. But one could just not specify the schema in their routing XML file. That could be a problem. I'll check that.