Quiz Stats - Normalize input

Ensure all input hashes are using symbol keys.

Closes CNVS-13158

TEST PLAN
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Code changes. Test plan is similar to
https://gerrit.instructure.com/#/c/35096/

Change-Id: Ie1db45e2a5875e410a758c0b2f14594345212b5d
Reviewed-on: https://gerrit.instructure.com/35098
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>