config->site_url(): remove useless cast

thanks to narfbg
diff --git a/system/core/Config.php b/system/core/Config.php
index 9e6d2cb..38bcd5c 100644
--- a/system/core/Config.php
+++ b/system/core/Config.php
@@ -245,7 +245,7 @@
 
 		if ($this->item('enable_query_strings') === FALSE)
 		{
-			$suffix = isset($this->config['url_suffix']) ? (string) $this->config['url_suffix'] : '';
+			$suffix = isset($this->config['url_suffix']) ? $this->config['url_suffix'] : '';
 
 			if ($suffix !== '')
 			{