Support multiple open order line with the same effective_date.

If 2 lines have the same effective date (catalog has a 1 second precision),
always return the validate and open first.

Commit: 02d06501

Changes applied from history:

http://git.erp5.org/gitweb/erp5.git/history/refs/heads/interaction-drop:/product/ERP5/Document/SubscriptionItem.py?js=1