Update cart.py

[scutarius]

Please update code in the Cart.py

[michaelavila]

@scutarius thanks for the pull-request! Could you provide some background information? Why you needed this? And how you'd go about testing what you've added? If you could add the unit tests yourself that would be awesome, but if that's not possible just a description will be good and we can go ahead and add the tests ourselves. Let me know what you think!

// cc @bmentges

[scutarius]

Excuse me, I have a bad knowledge of English, so I use a translator. In one project, a simple online store, when suddenly the buyer wants to change the quantity of the goods, and the buyer on the product order page, then it's better to update the quantity on this page. And modify the functional baskets is not bad :)
In my views.py when quantity> 0
quantity> 0: cart.update (product, quantity)
But if you enter 0
elif quantity == 0: cart.remove (product)

[michaelavila]

@scutarius thanks! We'll start looking at this. @bmentges is this something you can write some tests for? Let me know.