Merge pull request #21374 from rodzyn/protected_not_needed

Make `assert_index` private

actionpack/lib/action_dispatch/middleware/stack.rb

@@ -87,7 +87,7 @@ def build(app = Proc.new)
       middlewares.freeze.reverse.inject(app) { |a, e| e.build(a) }
     end
 
-  protected
+    private
 
     def assert_index(index, where)
       index = get_class index
@@ -96,8 +96,6 @@ def assert_index(index, where)
       i
     end
 
-    private
-
     def get_class(klass)
       if klass.is_a?(String) || klass.is_a?(Symbol)
         classcache = ActiveSupport::Dependencies::Reference