Fix deprecated ActionController::Base.relative_url_root call in mongrel command

Signed-off-by: NJoshua Peek <josh@joshpeek.com>

railties/lib/rails/mongrel_server/commands.rb

@@ -44,7 +44,7 @@ def mount_rails(prefix)
         env_location = "#{defaults[:cwd]}/config/environment"
         require env_location
 
-        ActionController::AbstractRequest.relative_url_root = defaults[:prefix]
+        ActionController::Base.relative_url_root = defaults[:prefix]
         uri prefix, :handler => Rails::MongrelServer::RailsHandler.new
       end
     end