[Nereid] Issue#24 Fix for Nereid CMS #3163

* Fixed the check_constraint method by using
  the right method
* Fixed selection field on Menu Item not saving
  if list is empty
* Wrote test

Project-ID: 989
Task-ID: 3163
Git Issue: https://github.com/openlabs/nereid-cms/issues/24
Merge pull request #28 from uditagarwal/2.8

[Nereid] Issue#24 Fix for Nereid CMS #3163