title: How to apply discount to basket line?
body: <p>New to Django-oscar and can't have answers on the official docs
  I'm trying to add a discount
  I tried this:</p>
  
  <pre><code>line, created = self.request.basket.add_product(product=product, quantity=serializer.validated_data['quantity'], options=options)
  line.discount(discount_value=20, affected_quantity=1)
  line.save()
  </code></pre>
  
  <p>and when I try to print <code>basket.total_excl_tax_excl_discounts</code> or <code>basket.total_excl_tax</code> I always get the same result (which is the total of basket no matter discounts I added)</p>
  
  <p>what am I missing ??</p>