handle caching of shards when the app adds serialized attributes
handle caching of shards when the app adds serialized attributes

test plan:
 * use switchman in an app that adds a serialized attribute to shard
 * user Shard.lookup(x)
 * the serialized attribute should behave correctly

Change-Id: I3b6ed7c3d5f2318d374cbfbbc575c452085dc7c9
Reviewed-on: https://gerrit.instructure.com/36115
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jacob Fugal <f862f167b85d41b225785c70d70808bc7337c1fe@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

instructure/switchman,ccutrer/switchman,instructure/switchman,krrg/switchman