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.

The only fix I can think of, is to clone the route, so that we don't have the same reference twice.