add stub for arel-style shard finder option

Change-Id: I0022ef0526013e6a79ea580fa9da1b10a6c44bd5
Reviewed-on: https://gerrit.instructure.com/18927
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

lib/stubs/shard.rb

@@ -92,7 +92,9 @@ ActiveRecord::Base.class_eval do
     end
   end
 
-  def shard
+  named_scope :shard, lambda { |shard| {} }
+
+  def shard(shard = nil)
     Shard.default
   end