Provide compatibility with old and new RailsAdmin for html_default_value/form_default_value

Recent changes created compatibility with new RailsAdmin but broke it with old RailsAdmin. New RailsAdmin does not work with Rails 3.x, so some of us can't upgrade. This simple fix _should_ restore functionality. 

I have tested it with old RailsAdmin (0.4.x), but someone should test it with new RailsAdmin just to be sure.