[12.0][IMP] account_invoice_refund_line_selection: prepare refund vals

[Abranes]

Call the Odoo base function _prepare_refund because this function implements that logic and some other modules inherit it to add more data.

For example the module account_invoice_pricelist in this same repository which adds pricelist_id to account.invoice and inherits _prepare_refund to pass the pricelist_id when you create a refund. With the current code we miss the pricelist when we create a refund but with this changes we preserve this value and all others.

[etobella]

It was done this way because there are some fields that needed to be recalcualted (like taxes, I am not sure if this will work in all cases 😭 )