Dont override routes when it's been set already

Could make it easier for engines to have their specific routeset for all
controller specs in the build. Currently we need to do it on a per class
basis

Good chance I might be missing something but is there a clear way to have a specific route set for all controllers in a build? I was trying to set it in a `before` hook on the configure block in spec_helper but realized the routeset is always overridden later on in controller_example_group.

Having trouble to bundle rspec-rails right now so couldn't run the specs here.