[REF] account_invoice_change_currency: Fixed get rate at first currency

convertion.

Before this commit, was returned the same currency that the invoice on
the onchange context, now is returned the currency recorded at the
messages with tracking values.

Author: hugho-ad

account_invoice_change_currency/models/account_change_currency.py

@@ -135,6 +135,7 @@ def get_last_currency_id(self, skip_update_currency=False):
     @api.multi
     def get_last_rate(self):
         self.ensure_one()
+        subtype_create_id = self.env.ref('account.mt_invoice_created')
         last_values = self.env['mail.tracking.value'].sudo().search([
             ('mail_message_id', 'in', self.message_ids.ids),
             ('field', 'in', ['rate', 'currency_id']),
@@ -146,6 +147,10 @@ def get_last_rate(self):
                                                 key=lambda r: r.field)
             return (self.currency_id.browse(currency_value.old_value_integer),
                     rate_value.old_value_float)
+        if (len(last_values) == 1 and last_values.mail_message_id.subtype_id ==
+                subtype_create_id):
+            return (self.currency_id.browse(last_values.new_value_integer),
+                    None)
         return self.currency_id.browse(None), None
 
     @api.multi