Headings have producline suffix of 80

This is not document anywhere, but 6308000000 is a good example. There
are two headings with this goods code. The one with product line suffix
of 10 is wrapping/grouping heading and one with 80 is real one.
Merge pull request #104 from alphagov/55703384-fix-missing-duties

55703384 fix missing duties

https://www.pivotaltracker.com/story/show/55703384