Reverts behavior change to UrlGenerator

I do not want to talk much about the behavior change and whether it makes sense or not because I think it does not matter in this situation.

The ``generate`` method is tagged with ``@api``, there is no security issue that was fixed. According to the rules set forth at http://symfony.com/doc/current/book/stable_api.html, the semantics of such a method must not be changed.

There is some more discussion in #6814 and the commit changing the behavior is this one: https://github.com/symfony/Routing/commit/c66d1f9de30fd1b6a86cca10dd79d12c9ba9ff25#diff-0