Mailgun: Fix lost attachments with non-ASCII filenames.

Workaround requests/requests#4652 (urllib3/urllib3#303), where
uploaded files in multipart/form-data are improperly given RFC 2231
encoded filenames. That format is not accepted by Mailgun's API (and is
prohibited by RFC 7578), resulting in the attachments being silently
dropped.

Fix is to patch up the multipart/form-data before posting to remove
the RFC 2231 encoding.

Fixes #125

tests/test_mailgun_integration.py

@@ -1,3 +1,7 @@
+# -*- coding: utf-8 -*-
+
+from __future__ import unicode_literals
+
 import os
 import logging
 import unittest
@@ -116,7 +120,7 @@ class MailgunBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
             track_opens=True,
         )
         message.attach("attachment1.txt", "Here is some\ntext for you", "text/plain")
-        message.attach("attachment2.csv", "ID,Name\n1,3", "text/csv")
+        message.attach("vedhæftet fil.csv", "ID,Name\n1,3", "text/csv")
         cid = message.attach_inline_image_file(sample_image_path(), domain=MAILGUN_TEST_DOMAIN)
         message.attach_alternative(
             "<div>This is the <i>html</i> body <img src='cid:%s'></div>" % cid,
@@ -151,7 +155,7 @@ class MailgunBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
         self.assertEqual(len(attachments), 2)  # because inline image shouldn't be an attachment
         self.assertEqual(attachments[0]["filename"], "attachment1.txt")
         self.assertEqual(attachments[0]["content-type"], "text/plain")
-        self.assertEqual(attachments[1]["filename"], "attachment2.csv")
+        self.assertEqual(attachments[1]["filename"], "vedhæftet fil.csv")
         self.assertEqual(attachments[1]["content-type"], "text/csv")
 
         # No other fields are verifiable from the event data.