Give positive attitude to method is_shipped which was always False.
Merge pull request #68 from yvaucher/8.0-fix-sale_sourced_by_line-is_delivered

8.0 sale_sourced_by_line - Fix is_shipped method
refactor duplicated checks for skipping and fix

If you look closely at the diff, you will see that if the product was
defined, the check on make_to_stock was never done. This happens most of
the time.
skip checks for non-stockable products
call my methods, this is not ruby