Improve usability of SimpleQuery when directly instanciated: automaticaly convert an "=" comparison operator on a None value into an "is" comparison operator.
Update code comment, as it is not about backward compatibility but about usability.
Add a test.


git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@27070 20353a03-c40f-0410-a6d1-a30d3c3de9de