instance_variables returns symbols, so we should use symbols in our list

actionpack/lib/abstract_controller/rendering.rb

@@ -58,7 +58,7 @@ def rendered_format
     DEFAULT_PROTECTED_INSTANCE_VARIABLES = %w(
       @_action_name @_response_body @_formats @_prefixes @_config
       @_view_context_class @_view_renderer @_lookup_context
-    )
+    ).map(&:to_sym)
 
     # This method should return a hash with assigns.
     # You can overwrite this configuration per controller.