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.
Removed stray </div> tag.
Merge pull request #502 from maikhoepfel/fix/producttemplate

Fix template error for product class.