Add test for ChallengeBody proxy behaviour

letsencrypt/acme/messages2_test.py

@@ -169,6 +169,9 @@ class ChallengeBodyTest(unittest.TestCase):
         from letsencrypt.acme.messages2 import ChallengeBody
         hash(ChallengeBody.from_json(self.jobj_from))
 
+    def test_proxy(self):
+        self.assertEqual('foo', self.challb.token)
+
 
 class AuthorizationTest(unittest.TestCase):
     """Tests for letsencrypt.acme.messages2.Authorization."""