[FixturesBundle] Fix for fixtures populator entity array

[virtualize]

If your fixtures contain an array of related entities like this

\App\SiteBundle\Entity\Pages\ArticlePage:
    article_{1..25}:
        categories: [@cat_1, @cat_4]
        ...

You end up with following error, as the entity references in this array are not treated as entities while populatoring: Expected value of type "Doctrine\Common\Collections\Collection|array" for association field "App\SiteBundle\Entity\Pages\ArticlePage#$categories", got "Kunstmaan\Fixtures Bundle\Loader\Fixture" instead.

This pull request takes care of this, although I guess there might be some prettier way...