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. Altering the template code 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.

dashboard/catalogue/views.py:

    class ProductCreateView(generic.CreateView):
        [...]
    
        def get_context_data(self, **kwargs):
            ctx = super(ProductCreateView, self).get_context_data(**kwargs)
            pclass = self.get_product_class()
            if 'stockrecord_form' not in ctx:
                ctx['stockrecord_form'] = StockRecordForm(pclass)
            if 'category_formset' not in ctx:
                ctx['category_formset'] = ProductCategoryFormSet()
            if 'image_formset' not in ctx:
                ctx['image_formset'] = ProductImageFormSet()
            ctx['title'] = _('Create new %s product') % pclass.name
            ctx['product_class'] = pclass
            return ctx
    
        def get_product_class(self):
            return ProductClass.objects.get(id=self.kwargs['product_class_id'])
    
        def get_form_kwargs(self):
            kwargs = super(ProductCreateView, self).get_form_kwargs()
            kwargs['product_class'] = self.get_product_class()
            return kwargs