oauth_google_login: support refresh when refresh_token is still valid
When refresh_token is still valid google's endpoint does not include the current refresh token in the response when refreshing the token, we need to keep the current one.
This fixes user logout every one hour.
That the old implementation before c3488a67 was doing something similar in https://github.com/googleapis/oauth2client/blob/v4.0.0/oauth2client/client.py#L781