fixes a bug in the LTI variable substitutor

test plan:

Add a custom variable with the value $Canvas.user.user and $Canvas.user.class.
These values should not be substituted when the tool launches .

fixes PLAT-311

Change-Id: Ifb64ebaeb0c732720d6c5312cbc595a9c6e9c21e
Reviewed-on: https://gerrit.instructure.com/28761
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>