ensure that Association#with_each_shard doesn't use loaded results

Change-Id: Id36a367ea5f135f87e548c64afe8b0c640b6acfc
Reviewed-on: https://gerrit.instructure.com/31443
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

Author: ccutrer

lib/stubs/shard.rb

@@ -125,7 +125,7 @@ def shard
   %w{HasManyAssociation HasManyThroughAssociation}.each do |klass|
     const_get(klass).class_eval do
       def with_each_shard(*shards)
-        scope = self
+        scope = self.scoped
         scope = yield(scope) if block_given?
         Array(scope)
       end