code: def setup_phase
        if options[:setup].respond_to?(:call)
          log :info, 'Setup endpoint detected, running now.'
          options[:setup].call(env)
        elsif options[:setup]
          log :info, 'Calling through to underlying application for setup.'
          setup_env = env.merge('PATH_INFO' => setup_path, 'REQUEST_METHOD' => 'GET')
          call_app!(setup_env)
        end
      end
docs: The setup phase looks for the `:setup` option to exist and,
   if it is, will call either the Rack endpoint supplied to the
   `:setup` option or it will call out to the setup path of the
   underlying application. This will default to `/auth/:provider/setup`.