Update Router.php

Fixed bug where urlForl remove all characters after an empty optional segment.

Ie.
//Old behavior
testroute: /partA/partB(/:partC)/partD(.html)
$url = $app->urlFor('testroute'); //Would become /partA/partB

//New behavior
testroute: /partA/partB(/:partC)/partD(.html)
$url = $app->urlFor('testroute'); //Would become /partA/partB/pardD.html