Fixes a regression caused by having mapping options controller and action as blank

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -152,7 +152,10 @@ def default_controller_and_action(to_shorthand=nil)
                 raise ArgumentError, "missing :action"
               end
 
-              { :controller => controller, :action => action }
+              { :controller => controller, :action => action }.tap do |hash|
+                hash.delete(:controller) if hash[:controller].blank?
+                hash.delete(:action)     if hash[:action].blank?
+              end
             end
           end