Update ServerShell to use provided documentRoot.

Fix ServerShell from using the wrong path to the routing script when
a custom document root is used. Thanks to Marko Marković for the patch.

Fixes #3909

lib/Cake/Console/Command/ServerShell.php

@@ -130,7 +130,7 @@ class ServerShell extends AppShell {
 			$this->_host,
 			$this->_port,
 			$this->_documentRoot,
-			WWW_ROOT . '/index.php'
+			$this->_documentRoot . '/index.php'
 		);
 
 		$port = ($this->_port == self::DEFAULT_PORT) ? '' : ':' . $this->_port;