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 <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>