Fix bug introduced in 79ddc2e8e8

Order#find_line_item_by_variant is used internally by spree she adding a
line_item to an order, we can't stop using that method's name if we want
to detect the correct line_item(cherry picked from commit 4d498a7037ac8cb8a61323e6c8d497596636f469)
Merge pull request #6 from geermc4/fix-mixed-products

Fix bug introduced in 79ddc2e8e8