Git commits in Moodle 3.1.5 by Tim Lock

commit dfbbb39a25618a49974fa914a7678b68cfe5e738
Author: Tim Lock <tim@netspot.com.au>
Date:   Tue, 17 Jan 2017 09:50:26 +0800
Tag:    v3.1.5

MDL-57668 mod_assign: Fix optional_param() form processing