inline only usage of Array#to_ics

Change-Id: Ife84767f08ba50978e43045054bb9906443c03f3
Reviewed-on: https://gerrit.instructure.com/30030
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Raphael Weiner <rweiner@pivotallabs.com>
Product-Review: Raphael Weiner <rweiner@pivotallabs.com>
QA-Review: Raphael Weiner <rweiner@pivotallabs.com>