avoid ruby 3.4 chilled string warning

Author: wlipa

Diff for: lib/rails_admin/config/configurable.rb

@@ -52,7 +52,7 @@ def register_instance_option(option_name, scope = self, &default)
           options = scope.instance_variable_get('@config_options') ||
                     scope.instance_variable_set('@config_options', {})
 
-          option_name = option_name.to_s
+          option_name = option_name.to_s.dup
           options[option_name] = nil
 
           # If it's a boolean create an alias for it and remove question mark