login method
Log a user in on-demand.
Implementation
Future login(AuthToken token, RequestContext req, ResponseContext res) async {
var user = await deserializer(token.userId);
_apply(req, res, token, user);
_onLogin.add(user);
if (allowCookie) {
_addProtectedCookie(res, 'token', token.serialize(_hs256));
}
}