Correct negation of voucher discounts

[mcdado]

Besides being more elegant and correct, this way enables developers to create custom behaviors with Cart Rules, where instead of offering discounts, they could increase the price. However, this change will not effect anybody else.

See for example these screenshots: Before:

After:

You can see how now the normal case is still a normal minus, while with our custom built override we have a positive value, that is they way it's meant to be.

Never mind the rounding to four decimals, that is a customization.

[mcdado]

:up:

[mcdado]

Anybody?