Before this change if only one of storage_url or auth_token were
supplied then rclone would overwrite both of them when authenticating.
This effectively meant you could supply both of them or none of them
only.
Now rclone still does the authentication to read the missing
storage_url or auth_token then afterwards re-writes the auth_token or
storage_url back to what the user desired.
Fixes#2464
842ed7d2a9