[FIX] Foriegn currency supplier inovice from picking use order price

Commit ff90f2181236868fc8e4e2bf041e7a656e2303ea changed invoice unit price calculation to use the value of `stock.move.price_unit` instead the PO price. This field value is always in company currency and
according to its definition, is a technical field used only for average price calculations.

The mentioned commit was made to fix bug: https://bugs.launchpad.net/openobject-addons/+bug/779064

This bug was based on a case where users change the unit price in the stock move. As of current Odoo 7.0, unit price in stock move is not displayed in any view and, based on definition, it should not be editable. So, the mentioned bug is actually invalid.

Instead of reverting the commit, this fix maintains the current behavior in case if a custom modules allows users to change move price.

Issue #4200