spec: fix gradebook_history_api_spec fragile failures
spec: fix gradebook_history_api_spec fragile failures

`date_string` collides with a method in text_helper

Change-Id: If7ab88a2abe5a0d890ae0b04059e92ad67b3be4c
Reviewed-on: https://gerrit.instructure.com/64319
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

roxolan/canvas-lms,HotChalk/canvas-lms,leftplusrightllc/canvas-lms,redconfetti/canvas-lms,venturehive/canvas-lms,leftplusrightllc/canvas-lms,grahamb/canvas-lms,leftplusrightllc/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,dgynn/canvas-lms,fronteerio/canvas-lms,narigone/canvas,redconfetti/canvas-lms,grahamb/canvas-lms,AndranikMarkosyan/lms,djbender/canvas-lms,sfu/canvas-lms,redconfetti/canvas-lms,AndranikMarkosyan/lms,redconfetti/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,narigone/canvas,SwinburneOnline/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,fronteerio/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,snehakachroo1/LSI_replica,sfu/canvas-lms,snehakachroo1/LSI_replica,HotChalk/canvas-lms,sfu/canvas-lms,AndranikMarkosyan/lms,HotChalk/canvas-lms,SwinburneOnline/canvas-lms,dgynn/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms,matematikk-mooc/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,dgynn/canvas-lms,roxolan/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,dgynn/canvas-lms,snehakachroo1/LSI_replica,fronteerio/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,SwinburneOnline/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,narigone/canvas,snehakachroo1/LSI_replica,grahamb/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,narigone/canvas,leftplusrightllc/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,AndranikMarkosyan/lms