Revocation: expect application/json (boulder#771).

acme/acme/client.py

@@ -454,8 +454,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
 
         """
         response = self.net.post(self.directory[messages.Revocation],
-                                 messages.Revocation(certificate=cert),
-                                 content_type=None)
+                                 messages.Revocation(certificate=cert))
         if response.status_code != http_client.OK:
             raise errors.ClientError(
                 'Successful revocation must return HTTP OK status')

acme/acme/client_test.py

@@ -360,7 +360,7 @@ class ClientTest(unittest.TestCase):
     def test_revoke(self):
         self.client.revoke(self.certr.body)
         self.net.post.assert_called_once_with(
-            self.directory[messages.Revocation], mock.ANY, content_type=None)
+            self.directory[messages.Revocation], mock.ANY)
 
     def test_revoke_bad_status_raises_error(self):
         self.response.status_code = http_client.METHOD_NOT_ALLOWED