fix write to slave in GradebooksController#show
fix write to slave in GradebooksController#show

fixes CNVS-2624

turns out that calling foo.build_bar touches bar if it already exists

Test plan:
  make sure gradebook download/upload still works in gb1

Change-Id: Id5bb09f3da8336de54eb576274f0a13e6c254b81
Reviewed-on: https://gerrit.instructure.com/26292
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: Amber Taniuchi <ef4bfd1e6d278f6bc7d4a478f02be2cb9581a0a5@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Cameron Matheson <1d572acbfa68c7c6e541c7b840d6b622e5c0dc91@instructure.com>

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