Add: postFilter() method

Will be invoked right after the controller action has been called.
Fix: return response instead of sending it

The problem: When calling redirect(), the response is sent and
terminated by exit. So there is no chance for postFilter() to be get
called.

This fix also influences the usage. Always return redirect()!

Prior, the following code was possible:
$this->redirect('/foo');
// Immediately redirects

Now:
return $this->redirect('/foo');
Refactor: use isset() with multiple args