ProductCreateUpdateView: Avoid AttributeError
ProductCreateUpdateView: Avoid AttributeError

DELETE requests were failing with an exception instead
of the appropriate HTTP 405 response.

Fixes https://github.com/django-oscar/django-oscar/issues/3429

django-oscar/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,solarissmoke/django-oscar,django-oscar/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,solarissmoke/django-oscar