[FIX] sale: _get_sale_order_line

When making several expenses for the same analytic account,
it can be possible that these expenses have the same product but different
price unit(e.g.:this is the case for plane ticket with a vairiable price
according to the time). Then the function "_get_sale_order_line" must check
for all the existing confirmed SO lines if there is one which matches with
the analytic account, the product and the price before creating a new SO line.

opw:658383