[FIX] point_of_sale: correctly copy the base_amount sign of fixed tax

All the POS has to do is copy what accounting does. In the case of a
fixed tax we should do (from account.tax._compute_amount):

return math.copysign(self.amount, base_amount) * quantity

where copysign(x, y) returns x with the sign of y.

When base_amount is 0 it's positive, not negative. This also correctly
handles a base_amount of -0, although it's something that we shouldn't
encounter in the POS.

opw-672118

addons/point_of_sale/static/src/js/models.js

@@ -1322,7 +1322,7 @@ exports.Orderline = Backbone.Model.extend({
     _compute_all: function(tax, base_amount, quantity) {
         if (tax.amount_type === 'fixed') {
             var ret = tax.amount * quantity;
-            return base_amount >= 0 ? ret : ret * -1;
+            return base_amount > 0 ? ret : ret * -1;
         }
         if ((tax.amount_type === 'percent' && !tax.price_include) || (tax.amount_type === 'division' && tax.price_include)){
             return base_amount * tax.amount / 100;