Merge PO with sale_dropshipping

Updates on sale_dropshipping to :
- do not merge orders with different destination addresses
- populate the destination address when merging two purchase orders

Original Launchpad MP : https://code.launchpad.net/~camptocamp/sale-wkfl/7.0-dropshipping-merge-po-lep/+merge/216756
I don't see special problems with that form, but in this case is simpler. You can simply put `res['dest_address_id'] = order.dest_address_id.id`, because it will returns False in case of browse_null.