Mailgun: disable non-ASCII attachment filename workaround when not needed

urllib3 v1.25 fixes non-ASCII filenames in multipart form data to be
RFC 5758 compliant by default, so our earlier workaround is no longer
needed. Disable the workaround if we detect that Requests is using a
fixed version of urllib3.

Closes #157

tests/mock_requests_backend.py

@@ -115,6 +115,13 @@ class RequestsBackendMockAPITestCase(SimpleTestCase, AnymailTestMixin):
         """Returns the auth sent to the mock ESP API"""
         return self.get_api_call_arg('auth', required)
 
+    def get_api_prepared_request(self):
+        """Returns the PreparedRequest that would have been sent"""
+        (args, kwargs) = self.mock_request.call_args
+        kwargs.pop('timeout', None)  # Session-only param
+        request = requests.Request(**kwargs)
+        return request.prepare()
+
     def assert_esp_not_called(self, msg=None):
         if self.mock_request.called:
             raise AssertionError(msg or "ESP API was called and shouldn't have been")

tests/test_mailgun_backend.py

@@ -182,12 +182,20 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         self.message.send()
 
         # Verify the RFC 7578 compliance workaround has kicked in:
-        data = self.get_api_call_data().decode("utf-8")
+        data = self.get_api_call_data()
+        if isinstance(data, dict):  # workaround not needed or used (but let's double check actual request)
+            workaround = False
+            prepared = self.get_api_prepared_request()
+            data = prepared.body
+        else:
+            workaround = True
+        data = data.decode("utf-8").replace("\r\n", "\n")
         self.assertNotIn("filename*=", data)  # No RFC 2231 encoding
         self.assertIn(u'Content-Disposition: form-data; name="attachment"; filename="Une pièce jointe.html"', data)
 
-        files = self.get_api_call_files(required=False)
-        self.assertFalse(files)  # files should have been moved to formdata body
+        if workaround:
+            files = self.get_api_call_files(required=False)
+            self.assertFalse(files)  # files should have been moved to formdata body
 
     def test_rfc_7578_compliance(self):
         # Check some corner cases in the workaround that undoes RFC 2231 multipart/form-data encoding...
@@ -214,7 +222,12 @@ class MailgunBackendStandardEmailTests(MailgunBackendMockAPITestCase):
         self.message.attach(u"besked med vedhæftede filer", forwarded_message, "message/rfc822")
         self.message.send()
 
-        data = self.get_api_call_data().decode("utf-8").replace("\r\n", "\n")
+        data = self.get_api_call_data()
+        if isinstance(data, dict):  # workaround not needed or used (but let's double check actual request)
+            prepared = self.get_api_prepared_request()
+            data = prepared.body
+        data = data.decode("utf-8").replace("\r\n", "\n")
+
         # Top-level attachment (in form-data) should have RFC 7578 filename (raw Unicode):
         self.assertIn(
             u'Content-Disposition: form-data; name="attachment"; filename="besked med vedhæftede filer"', data)