[FIX] account: exchange rate of price_subtotal_signed

The field `price_subtotal_signed` contains the price converted in the
company currency. However, we should make sure to use the exchange rate
of the invoice date for its computation.

An use problematic use case is the creation of a deferred revenue from
an invoice (commit 7fe89d08b3).

opw-702352

addons/account/models/account_invoice.py

@@ -1105,7 +1105,8 @@ class AccountInvoiceLine(models.Model):
 
     @api.one
     @api.depends('price_unit', 'discount', 'invoice_line_tax_ids', 'quantity',
-        'product_id', 'invoice_id.partner_id', 'invoice_id.currency_id', 'invoice_id.company_id')
+        'product_id', 'invoice_id.partner_id', 'invoice_id.currency_id', 'invoice_id.company_id',
+        'invoice_id.date_invoice')
     def _compute_price(self):
         currency = self.invoice_id and self.invoice_id.currency_id or None
         price = self.price_unit * (1 - (self.discount or 0.0) / 100.0)
@@ -1114,7 +1115,7 @@ class AccountInvoiceLine(models.Model):
             taxes = self.invoice_line_tax_ids.compute_all(price, currency, self.quantity, product=self.product_id, partner=self.invoice_id.partner_id)
         self.price_subtotal = price_subtotal_signed = taxes['total_excluded'] if taxes else self.quantity * price
         if self.invoice_id.currency_id and self.invoice_id.company_id and self.invoice_id.currency_id != self.invoice_id.company_id.currency_id:
-            price_subtotal_signed = self.invoice_id.currency_id.compute(price_subtotal_signed, self.invoice_id.company_id.currency_id)
+            price_subtotal_signed = self.invoice_id.currency_id.with_context(date=self.invoice_id.date_invoice).compute(price_subtotal_signed, self.invoice_id.company_id.currency_id)
         sign = self.invoice_id.type in ['in_refund', 'out_refund'] and -1 or 1
         self.price_subtotal_signed = price_subtotal_signed * sign