[REF] account: hook for currency rate

Commit 74e4130f5f makes sure to make the accounting date prevail on
the invoice date when it comes to currency conversion.

However, the choice of date is not universally recognized. Softwares
(and accountants) might use either the accounting date or the invoice
date.

Therefore, we provide a hook se the behavior of the software can easily
be changed if necessary.

opw-783372

addons/account/models/account_invoice.py

@@ -700,7 +700,7 @@ class AccountInvoice(models.Model):
         total_currency = 0
         for line in invoice_move_lines:
             if self.currency_id != company_currency:
-                currency = self.currency_id.with_context(date=self.date or self.date_invoice or fields.Date.context_today(self))
+                currency = self.currency_id.with_context(date=self._get_currency_rate_date() or fields.Date.context_today(self))
                 if not (line.get('currency_id') and line.get('amount_currency')):
                     line['currency_id'] = currency.id
                     line['amount_currency'] = currency.round(line['price'])
@@ -848,7 +848,7 @@ class AccountInvoice(models.Model):
             if inv.payment_term_id:
                 totlines = inv.with_context(ctx).payment_term_id.with_context(currency_id=company_currency.id).compute(total, inv.date_invoice)[0]
                 res_amount_currency = total_currency
-                ctx['date'] = inv.date or inv.date_invoice
+                ctx['date'] = inv._get_currency_rate_date()
                 for i, t in enumerate(totlines):
                     if inv.currency_id != company_currency:
                         amount_currency = company_currency.with_context(ctx).compute(t[1], inv.currency_id)
@@ -1029,6 +1029,9 @@ class AccountInvoice(models.Model):
         copy_fields = ['company_id', 'user_id', 'fiscal_position_id']
         return self._get_refund_common_fields() + self._get_refund_prepare_fields() + copy_fields
 
+    def _get_currency_rate_date(self):
+        return self.date or self.date_invoice
+
     @api.model
     def _prepare_refund(self, invoice, date_invoice=None, date=None, description=None, journal_id=None):
         """ Prepare the dict of values to create the new refund from the invoice.