Avoid sorting an Array including objects from different Classes

addresses "ArgumentError: comparison of VerySpecialComment with SpecialComment failed" in ActiveRecord::DelegationRelationTest#test_#sort!_delegation_is_deprecated

activerecord/test/cases/relation/delegation_test.rb

@@ -64,7 +64,7 @@ class DelegationRelationTest < DelegationTest
     fixtures :comments
 
     def target
-      Comment.where.not(body: nil)
+      Comment.where(body: 'Normal type')
     end
 
     [:map, :collect].each do |method|