fix automatic activation of DJ category
fix automatic activation of DJ category

refs CNVS-14629

test plan:
 * have three shards
 * set the shard 2's delayed_job_shard_id to shard 3
 * Shard.find(2).activate { Shard.current(:delayed_jobs) }
   should return shard 3, not the default shard

Change-Id: I296885c331b31a59214314a72a74f5e4b1198d0b
Reviewed-on: https://gerrit.instructure.com/38895
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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