Simplify RedirectHandler

[g105b]

RedirectHandler should construct with a nullable callback function. If it's not supplied, then continue using the header() call by default, otherwise call the callback.

This will allow passing in the Gt\Http\Response's redirect function, like this:

new Authenticator(
	$this->config->getString("authwave.api_key"),
	$serverInfo->getRequestUri(),
	$this->config->getString("authwave.host"),
	$awSession,
	$response->redirect(...),
);