Fix template error for product class.

templates/oscar/dashboard/catalogue/product_update.html tried to render
the product class via {{ product.product_class }}. 'product' is not
in the template context, but 'product_class' is, which gives the
expected result.

The bug is currently visible in the Oscar Sandbox.

Note: This was spotted because I have [TEMPLATE_STRING_IF_INVALID](https://docs.djangoproject.com/en/dev/ref/settings/#template-string-if-invalid) set to something obnoxious. Recommended.