inline only usage of Array#count_per

Change-Id: I79d32eedcf8c00259777ed04df605905ca3f685f
Reviewed-on: https://gerrit.instructure.com/30026
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>