Adds a test for #18584.

git-svn-id: http://svn.redmine.org/redmine/trunk@13722 e93f8b46-1217-0410-a6f0-8f06a7374b81
Skip some validations if attribute did not change.

git-svn-id: http://svn.redmine.org/redmine/trunk@13723 e93f8b46-1217-0410-a6f0-8f06a7374b81