Support HTTP 405 response code

[jtojnar]

This will allow us to have code like this:

$router->set404(function($handledByOtherMethod) {
    if ($handledByOtherMethod) {
        header('HTTP/1.1 405 Method Not Allowed');
    } else {
        header('HTTP/1.1 404 Not Found');
    }
    // ... do something special here
});

[oskarniziol]

Good idea!