[CoreBundle] Enable override of routing config

[QuingKhaos]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	n/a
Fixed tickets	n/a
License	MIT
Doc PR	n/a

I wanted to add the paginatate parameter to the sylius key for the generated routes. But without the change the configuration merge doesn't preserve keys and looked like the following:

sylius_core:
    routing:
        %sylius.model.taxon.class%:
            field: permalink
            prefix: /t
            defaults:
                controller: sylius.controller.product:indexByTaxonAction
                sylius:
                    template: SyliusWebBundle:Frontend/Product:indexByTaxon.html.twig
                0:
                    paginate: 9

With this change the configs get correctly merged to this:

sylius_core:
    routing:
        %sylius.model.taxon.class%:
            field: permalink
            prefix: /t
            defaults:
                controller: sylius.controller.product:indexByTaxonAction
                sylius:
                    template: SyliusWebBundle:Frontend/Product:indexByTaxon.html.twig
                    paginate: 9

[stloyd]

@pjedrzejewski @Arn0d Looks good for me...

[pjedrzejewski]

👍