Backout last change, save_cookie by itself does that

flask/sessions.py

@@ -169,10 +169,9 @@ class SecureCookieSessionInterface(SessionInterface):
     def save_session(self, app, session, response):
         expires = self.get_expiration_time(app, session)
         domain = self.get_cookie_domain(app)
-        if not session:
+        if session.modified and not session:
-            if session.modified:
             response.delete_cookie(app.session_cookie_name,
                                    domain=domain)
-            return
+        else:
             session.save_cookie(response, app.session_cookie_name,
                                 expires=expires, httponly=True, domain=domain)