Fix country search and add test to check it

mail_tracking_mailgun/models/mail_tracking_email.py

@@ -18,7 +18,7 @@ class MailTrackingEmail(models.Model):
         country = False
         if country_code:
             country = self.env['res.country'].search([
-                ('code', '=', country_code.capitalize()),
+                ('code', '=', country_code.upper()),
             ])
         if country:
             return country.id

mail_tracking_mailgun/tests/test_mailgun.py

@@ -154,6 +154,7 @@ class TestMailgun(TransactionCase):
         self.assertEqual(event.ua_family, ua_family)
         self.assertEqual(event.ua_type, ua_type)
         self.assertEqual(event.mobile, False)
+        self.assertEqual(event.user_country_id.code, 'US')
 
     # https://documentation.mailgun.com/user_manual.html#tracking-clicks
     def test_event_clicked(self):