Provide relative urls in redirects and not absolute ones

Assigning to myself as I'm triaging all RTBC issues.

Per #13, I think we can just unconditionally remove the setAbsolute()

That said, this is not consistent with how core is doing it. Both \Drupal\Core\Controller\ControllerBase::redirect() and \Drupal\Core\Form\FormSubmitter::redirectForm() for example are always doing absolute redirects, I'm unsure about going against core here.

Convert to MR, rebase with 8.x-1.x-dev