fixed tiny bug where assignments.get('assign_date') was undefined

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@876926 b8457f37-d9ea-0310-8a92-e5e31aec5664
stable tag v2.1.3

git-svn-id: https://plugins.svn.wordpress.org/an-gradebook/trunk@876927 b8457f37-d9ea-0310-8a92-e5e31aec5664