Don't need to check if the scope respond to call

We are checking this when defining the default scope and raising an
ArgumentError

activerecord/lib/active_record/scoping/default.rb

@@ -100,11 +100,7 @@ def build_default_scope # :nodoc:
           elsif default_scopes.any?
             evaluate_default_scope do
               default_scopes.inject(relation) do |default_scope, scope|
-                if !scope.is_a?(Relation) && scope.respond_to?(:call)
-                  default_scope.merge(unscoped { scope.call })
-                else
-                  default_scope.merge(scope)
-                end
+                default_scope.merge(unscoped { scope.call })
               end
             end
           end

activerecord/test/cases/associations/nested_through_associations_test.rb

@@ -371,7 +371,7 @@ def test_joins_and_includes_from_through_models_not_included_in_association
     prev_default_scope = Club.default_scopes
 
     [:includes, :preload, :joins, :eager_load].each do |q|
-      Club.default_scopes = [Club.send(q, :category)]
+      Club.default_scopes = [proc { Club.send(q, :category) }]
       assert_equal categories(:general), members(:groucho).reload.club_category
     end
   ensure

activerecord/test/cases/dup_test.rb

@@ -126,7 +126,7 @@ def test_dup_validity_is_independent
 
     def test_dup_with_default_scope
       prev_default_scopes = Topic.default_scopes
-      Topic.default_scopes = [Topic.where(:approved => true)]
+      Topic.default_scopes = [proc { Topic.where(:approved => true) }]
       topic = Topic.new(:approved => false)
       assert !topic.dup.approved?, "should not be overridden by default scopes"
     ensure