Fixes preload_scope when association is polymorphic

Presently, the `preload_scope` check fails because
`model.reflect_on_association(assocation).klass` will never be valid for
polymorphic associations.

This raises an ArgumentError if the association is polymorphic and
preload_scope is applied, but in all other ways it behaves the same.e

lib/graphql/preload/loader.rb

@@ -39,7 +39,10 @@ def perform(records)
       end
 
       private def preload_scope
-        scope if scope.try(:klass) == model.reflect_on_association(association).klass
+        return nil unless scope
+        reflection = model.reflect_on_association(association)
+        raise ArgumentError, 'Cannot specify preload_scope for polymorphic associations' if reflection.polymorphic?
+        scope if scope.try(:klass) == reflection.klass
       end
 
       private def validate_association