Supporting filter_backends option for ProductRetrieveView

[execut]

Hello! I think there is a flaw in ProductRetrieveView when looking for a product. The product search does not include the current CMS page. As a result, the product can be opened by a link that contains someone else's category. For example, we have an apple product and it is in the fruits category and is located under the /fruits/apple link. But because of this flaw, it can be opened for another existing category of vegetables: /vegetables/apple. This is because there is no filtering on the current CMS page in the product queryset here:

django-shop/shop/views/catalog.py

Lines 310 to 321 in 13d9a77

	def get_object(self):
	if not hasattr(self, '_product'):
	assert self.lookup_url_kwarg in self.kwargs
	filter_kwargs = {
	'active': True,
	self.lookup_field: self.kwargs[self.lookup_url_kwarg],
	}
	if hasattr(self.product_model, 'translations'):
	filter_kwargs.update(translations__language_code=get_language_from_request(self.request))
	queryset = self.product_model.objects.filter(self.limit_choices_to, **filter_kwargs)
	self._product = get_object_or_404(queryset)
	return self._product

How about add to ProductRetrieveView ability support of backend filters like CMSPagesFilterBackend?:

filter_backends = [CMSPagesFilterBackend]
filter_backends.extend(api_settings.DEFAULT_FILTER_BACKENDS)
return [
    ...
    url(r'^(?P<slug>[\w-]+)', ProductRetrieveView.as_view(
        filter_backends=filter_backends,
    )),