Support checking individual model attribute status

lib/liudesk_cmdb/model.rb

@@ -261,7 +261,9 @@ module LiudeskCMDB
       @data.merge!(attrs.select { |k, _| self.class.fields.key? k })
     end
 
-    def changed?
+    def changed?(key = nil)
+      return @data[key] != @old_data[key] if key
+
       @data != @old_data
     end