reuse the same frame so we don't need a singleton hash

We just want to augment the scope level, not the frame itself, so just
copy the frame to the new scope object.

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -1974,9 +1974,8 @@ def new(hash)
           self.class.new hash, self, scope_level
         end
 
-        EMPTY_HASH = {}.freeze
         def new_level(level)
-          self.class.new(EMPTY_HASH, self, level)
+          self.class.new(frame, self, level)
         end
 
         def [](key)