[FIX] stock_account: return product at purchased price

For FIFO and standard price, return product at the purchased price instead of
the cost or current standard price.
Not applied for average as returning a purchased product do not recompute
the average price and would lead to an inconsistency in the stock value.
opw 615263
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32