[FIX] point_of_sale: correct sign of fixed tax
[FIX] point_of_sale: correct sign of fixed tax

Oversight of e8b965f93c3eeb541aba7b6a435193a9008516ac. Although the
previous commit gave the correct behaviour for a negative fixed tax with
a base_amount of 0, it was still incorrect for positive fixed taxes with
a base_amount of 0.

This rewrites the code to more closely resemble the relevant code of
account.tax._compute_amount() solving the above mentioned issue.

ygol/odoo,hip-odoo/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,storm-computers/odoo,Elico-Corp/odoo_OCB,dfang/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,laslabs/odoo,bplancher/odoo,sysadminmatmoz/OCB,ygol/odoo,bplancher/odoo,dfang/odoo,dfang/odoo,microcom/odoo,Elico-Corp/odoo_OCB,microcom/odoo,laslabs/odoo,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,ygol/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,ygol/odoo,dfang/odoo,laslabs/odoo,microcom/odoo,microcom/odoo,stephen144/odoo,microcom/odoo,hip-odoo/odoo,stephen144/odoo,stephen144/odoo,stephen144/odoo,storm-computers/odoo,hip-odoo/odoo,storm-computers/odoo,hip-odoo/odoo,stephen144/odoo,dfang/odoo,microcom/odoo,laslabs/odoo,bplancher/odoo,dfang/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,ygol/odoo,laslabs/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,storm-computers/odoo,bplancher/odoo,sysadminmatmoz/OCB,storm-computers/odoo,bplancher/odoo,ygol/odoo,stephen144/odoo