Support controllers out of bundles

Need this to support [bundleless application code][1]. Currently using [my own bundle][2] for that, but would be nice to have this in the bundle itself.

[1]: http://elnur.pro/symfony-without-bundles/
[2]: https://github.com/elnur/ElnurTemplateGuesserBundle
`return` is not needed here, see https://github.com/symfony/HttpFoundation/commit/2ed84edfce6f85999818e8c4177a6a80e8135308.