[FIX] account: invoice has not the same currency as the purchase order

Used case:

-Configure company in EUR, and user in multi-currency
-Create a purchase order in USD and confirm it
-Click on the smart button "0 invoices" then click on "create"

Result before the fix:

The system created an invoice in EUR, currency of the company,
instead of in USD, currency of the purchase order.

Result after the fix:

The system created an invoice in USD, the currency of the related purchase order.

opw:677568

addons/account/models/account_invoice.py

@@ -69,6 +69,9 @@ class AccountInvoice(models.Model):
 
     @api.model
     def _default_currency(self):
+        if 'default_purchase_id' in self.env.context:
+            purchase_id = self.env.context['default_purchase_id']
+            return self.env['purchase.order'].browse(purchase_id).currency_id
         journal = self._default_journal()
         return journal.currency_id or journal.company_id.currency_id