Well actually

We don't need stripping after all.

acme/acme/client.py

@@ -258,7 +258,7 @@ class Client(object):  # pylint: disable=too-many-instance-attributes
         :rtype: `datetime.datetime`
 
         """
-        retry_after = response.headers.get('Retry-After', str(default)).strip()
+        retry_after = response.headers.get('Retry-After', str(default))
         try:
             seconds = int(retry_after)
         except ValueError:

acme/acme/client_test.py

@@ -205,11 +205,6 @@ class ClientTest(unittest.TestCase):
             datetime.datetime(2015, 3, 27, 0, 0, 10),
             self.client.retry_after(response=self.response, default=10))
 
-        self.response.headers['Retry-After'] = '20 '
-        self.assertEqual(
-            datetime.datetime(2015, 3, 27, 0, 0, 20),
-            self.client.retry_after(response=self.response, default=10))
-
         # wrong date -> ValueError
         dt_mock.datetime.side_effect = datetime.datetime
         self.response.headers['Retry-After'] = "Tue, 116 Feb 2016 11:50:00 MST"