fix sharding array coercion

This matches the behavior of the rails2 sharding stuff.  Specifically,
it doesn't fail when the return value of the block is true

Change-Id: I3f165d8c896bc038bf57908356b15c06dd76591b
Reviewed-on: https://gerrit.instructure.com/30081
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>