remove reliance on __toString()

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | [![Build Status](https://travis-ci.org/symfony-cmf/ContentBundle.png?branch=remove_to_string)](https://travis-ci.org/symfony-cmf/ContentBundle)
| Fixed tickets | https://github.com/symfony-cmf/RoutingBundle/issues/163
| License       | MIT