Fixed bug causing CSRF token not to rotate on login.

Thanks Gavin McQuillan for the report.

django/middleware/csrf.py

@@ -56,7 +56,10 @@ def rotate_token(request):
     Changes the CSRF token in use for a request - should be done on login
     for security purposes.
     """
-    request.META["CSRF_COOKIE"] = _get_new_csrf_key()
+    request.META.update({
+        "CSRF_COOKIE_USED": True,
+        "CSRF_COOKIE": _get_new_csrf_key(),
+    })
 
 
 def _sanitize_token(token):