Expand API for step2_exchange

In python 3, the code received from the browser will be binary instead of a string. Expand the API for oauth_flow.step2_exchange(code) to allow this value to be passed as is rather than decoding.

for example:
credentials = self.flow.step2_exchange(b'some random code')

test:
tox -e py34  -- tests.test_client:OAuth2WebServerFlowTest.test_exchange_success_binary_code

resolves:  #443, #446

oauth2client/client.py

@@ -2069,7 +2069,7 @@ def step2_exchange(self, code=None, http=None, device_flow_info=None):
 
         if code is None:
             code = device_flow_info.device_code
-        elif not isinstance(code, six.string_types):
+        elif not isinstance(code, (six.string_types, six.binary_type)):
             if 'code' not in code:
                 raise FlowExchangeError(code.get(
                     'error', 'No code was supplied in the query parameters.'))

tests/test_client.py

@@ -79,6 +79,7 @@
 from oauth2client.client import save_to_well_known_file
 from oauth2client.clientsecrets import _loadfile
 from oauth2client.service_account import ServiceAccountCredentials
+from oauth2client._helpers import _to_bytes
 
 __author__ = '[email protected] (Joe Gregorio)'
 
@@ -1201,6 +1202,26 @@ def test_exchange_success(self):
         self.assertEqual('dummy_revoke_uri', credentials.revoke_uri)
         self.assertEqual(set(['foo']), credentials.scopes)
 
+    def test_exchange_success_binary_code(self):
+        binary_code = b'some random code'
+        access_token = 'SlAV32hkKG'
+        expires_in = '3600'
+        refresh_token = '8xLOxBtZp8'
+        revoke_uri = 'dummy_revoke_uri'
+
+        payload = ('{'
+                   '  "access_token":"' + access_token + '",'
+                   '  "expires_in":' + expires_in + ','
+                   '  "refresh_token":"' + refresh_token + '"'
+                   '}')
+        http = HttpMockSequence([({'status': '200'}, _to_bytes(payload))])
+        credentials = self.flow.step2_exchange(binary_code, http=http)
+        self.assertEqual(access_token, credentials.access_token)
+        self.assertIsNotNone(credentials.token_expiry)
+        self.assertEqual(refresh_token, credentials.refresh_token)
+        self.assertEqual(revoke_uri, credentials.revoke_uri)
+        self.assertEqual(set(['foo']), credentials.scopes)
+
     def test_exchange_dictlike(self):
         class FakeDict(object):
             def __init__(self, d):