always cast `name` to a symbol, and never to_s it

All callers of `action_path` interpolate the return value in to a
string, so there is no need for the method to to_s it.  to_sym on a
symbol will return the same symbol, though I think `action_path` may
always be called with a symbol so this might not be necessary.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1743,8 +1743,7 @@ def path_for_action(action, path) #:nodoc:
           end
 
           def action_path(name) #:nodoc:
-            name = name.to_sym if name.is_a?(String)
-            @scope[:path_names][name] || name.to_s
+            @scope[:path_names][name.to_sym] || name
           end
 
           def prefix_name_for_action(as, action) #:nodoc: