Set picking type on PO generated from sourcing with FA

This bridge with onchanges on purchase_delivery_address
and make it consistant with purchase_requisition_delivery_address
@foutoucour can you please elaborate more? 

IMO we can have both 
- "real" isolated unit tests (setup - run one method - assert on the result)
- integration/story-like tests

In the core there are only integration ones, but that doesn't mean we shouldn't write isolated ones. I personally am for isolation wherever possible, but I'd rather avoid that big debate between isolation and integration and accept both styles.