Tests: work around Mailgun event reporting change

In the live integration tests, work around Mailgun's recent change
to how inline attachments are reported in "accepted" events data.

(But keep it working either way, since it seems likely this change was
unintentional—other event types still exclude inline attachments from
`event.message.attachments`.)

Fixes #172

tests/test_mailgun_integration.py

@@ -151,7 +151,13 @@ class MailgunBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
         self.assertEqual(headers["subject"], "Anymail Mailgun all-options integration test")
 
         attachments = event["message"]["attachments"]
-        self.assertEqual(len(attachments), 2)  # because inline image shouldn't be an attachment
+        if len(attachments) == 3:
+            # The inline attachment shouldn't be in the event message.attachments array,
+            # but sometimes is included for the accepted event (see #172)
+            inline_attachment = attachments.pop(0)
+            self.assertEqual(inline_attachment["filename"], cid)
+            self.assertEqual(inline_attachment["content-type"], "image/png")
+        self.assertEqual(len(attachments), 2)
         self.assertEqual(attachments[0]["filename"], "attachment1.txt")
         self.assertEqual(attachments[0]["content-type"], "text/plain")
         self.assertEqual(attachments[1]["filename"], "vedhæftet fil.csv")