I installed gnome-session 3.18.1.2-1ubuntu1.16.04.2 and gnome-settings-daemon 3.18.2-0ubuntu3.1 from xenial-proposed, accomplished the test case, and can confirm that the variable assignments work as expected.
I installed gnome-session 3.18.1. 2-1ubuntu1. 16.04.2 and gnome-settings- daemon 3.18.2-0ubuntu3.1 from xenial-proposed, accomplished the test case, and can confirm that the variable assignments work as expected.