contextId comes in as a string, not number

Change-Id: Iea294a0c72ef0828568b155dd3348862989599a8
Reviewed-on: https://gerrit.instructure.com/39156
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>