Modify stack update method
Modify stack update method

Do not use instance.__dict__.update(), as that may not apply changes to
attributes inherited from parent classes (as is the case with Stack).

hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock