[FIX] account: Making a refund with modified taxes

Step to reproduce the bug:
1.Create a product with a customer tax(15%) for a price=100
2.Create a customer invoice with this product
3.Change the amount of the tax to 10(instead of 15)
4.Validate the invoice
5.Click on "Refund invoice"
6.Select the refund method "Modify: create refund, reconcile
and create a new draft invoice"

Bug the new draft invoice has 15 as amount of tax

Now all the taxes from the invoice when preparing the refund in function
_prepare_refund, in this it keeps all the modified taxes.

opw:726491