Don't keep extra id_token info in token_response.

This tweaks `Flow.step2_exchange` to only preserve the raw `id_token` response
in the generated `Credentials`, and updates `CHANGELOG`.

Author: craigcitro

CHANGELOG

@@ -1,3 +1,8 @@
+v.next
+
+  Change `flow.step2_exchange` to preserve the raw `id_token` in the
+	`token_response` field.
+
 v1.3.2
   Version 1.3.2
 
@@ -8,7 +13,7 @@ v1.3.1
   Version 1.3.1
 
   Quick bugfix for bad error handling in from_json.
-	
+
 v1.3
   Version 1.3
 
@@ -31,7 +36,7 @@ v1.3
   - Better logging for missing `refresh_token` in server replies (#21).
   - Support `login_hint` (#18, thanks @jay0lee).
   - Better overwrite options in `django_orm.Storage`. (#2, thanks
-	@lraccomando). 
+	@lraccomando).
 
 
 v1.2

oauth2client/client.py

@@ -1838,14 +1838,14 @@ def step2_exchange(self, code=None, http=None, device_flow_info=None):
 
       extracted_id_token = None
       if 'id_token' in d:
-        d['extracted_id_token'] = _extract_id_token(d['id_token'])
+        extracted_id_token = _extract_id_token(d['id_token'])
 
       logger.info('Successfully retrieved access token')
       return OAuth2Credentials(access_token, self.client_id,
                                self.client_secret, refresh_token, token_expiry,
                                self.token_uri, self.user_agent,
                                revoke_uri=self.revoke_uri,
-                               id_token=d.get('extracted_id_token', None),
+                               id_token=extracted_id_token,
                                token_response=d)
     else:
       logger.info('Failed to retrieve access token: %s', content)