Update grant_type for AssertionCredentials.

Reviewed in https://codereview.appspot.com/7300056/.

oauth2client/client.py

@@ -854,7 +854,7 @@ class AssertionCredentials(OAuth2Credentials):
     body = urllib.urlencode({
         'assertion_type': self.assertion_type,
         'assertion': assertion,
-        'grant_type': 'assertion',
+        'grant_type': 'urn:ietf:params:oauth:grant-type:jwt-bearer',
         })
 
     return body

tests/test_oauth2client.py

@@ -289,6 +289,8 @@ class TestAssertionCredentials(unittest.TestCase):
     body = urlparse.parse_qs(self.credentials._generate_refresh_request_body())
     self.assertEqual(self.assertion_text, body['assertion'][0])
     self.assertEqual(self.assertion_type, body['assertion_type'][0])
+    self.assertEqual('urn:ietf:params:oauth:grant-type:jwt-bearer',
+                     body['grant_type'][0])
 
   def test_assertion_refresh(self):
     http = HttpMockSequence([