[FIX] hr_expense: tax included computation

When a tax included has a several childs, the source line should not be duplicated for each tax.
Without the patch, the actual product price (without taxes) would be (total - tax amount) for the last tax only (and not the sum of each tax). (opw 609104)
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-linux64