[trunk] incorrect stock accounting valuations using standard cost

in purchase module stock.py the method _get_reference_accounting_values_for_valuation is incorrectly overridden.

to reproduce.
Set a product as standard cost of $1.00
Raise a purchase order for this product at $1.50 * 10

Expected entries
Stock dr $10
Stock Input cr $10

Results
$15 in each.

if using standard cost the correct valuation at receipting time is the products standard cost * quantity. this is the whole point of using standard cost in the first place. However this override cause the stock valuation calculation to become po line price * qty. When products are sold downstream this will cause an error in stock valuation.

The line
if move.product_id.cost_method != 'average' or not move.price_unit:

should be changed, probably to

if move.product_id.cost_method == 'average' and not move.price_unit:

but in truth, this function could just be deleted entirely as there is no condition where this should evaluate to true, and even if it did, it is safer to take the standard_price which already occurs than just assume that the po_line is the correct price as the scale of potential errors is significantly smaller with the former and po's have no accounting entries.

In any case, with standard accounting it makes no difference, and with anglosaxon the price difference is picked up at invoice. Note that this bug is a root cause of the symptons of bug 922077