udpated xss_clean() in the security helper to pass $is_image instead of the deprecated $charset

http://codeigniter.com/bug_tracker/bug/6706/
diff --git a/system/helpers/security_helper.php b/system/helpers/security_helper.php
index 0e2ba78..4dbbf72 100644
--- a/system/helpers/security_helper.php
+++ b/system/helpers/security_helper.php
@@ -32,15 +32,15 @@
  *
  * @access	public
  * @param	string
- * @param	string	the character set of your data
+ * @param	bool	whether or not the content is an image file
  * @return	string
  */	
 if ( ! function_exists('xss_clean'))
 {
-	function xss_clean($str, $charset = 'ISO-8859-1')
+	function xss_clean($str, $is_image = FALSE)
 	{
 		$CI =& get_instance();
-		return $CI->input->xss_clean($str, $charset);
+		return $CI->input->xss_clean($str, $is_image);
 	}
 }