added failing test for double prefixes

This Pull Request should fix a bug introduced with https://github.com/FriendsOfSymfony/FOSRestBundle/pull/761

The same route is referenced at https://github.com/FriendsOfSymfony/FOSRestBundle/pull/761/files#diff-4f168cae49074c5f30b4abcccc85c281R558 twice, which results in a double prefixing.

This problem only occurs if a prefix is defined at the routing.yml file. The code at https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Routing/RouteCollection.php#L158 is making the code from my pull request fail. It iterates over all routes, and since two of them are referring the same object, the prefix is appended twice.