Fixing issue with Searchlogic-form-objects causing Formtastic to blow…

… up. Now works in my test-project - confusing to spec this one just for a certain plugin. Closing formtastic#155.

lib/formtastic.rb

@@ -311,7 +311,7 @@ def commit_button(*args)
       options = args.extract_options!
       text = options.delete(:label) || args.shift
 
-      if @object
+      if @object && @object.respond_to?(:new_record?)
         key = @object.new_record? ? :create : :update
         object_name = @object.class.human_name
       else