Addons add a + sign even if the value is negative - fix it

[Valentina R]

It was like this before the big Product Addons change.

Make it so that if the addon value is negative, to not append the plus sign.

Changes need to be made :

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Reported here.

[Valentina R]

SOLUTION: add one line and remove the default + sign:

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[Željan Topić]

Fixed with 4.18.0