Fix #346

When ['global_xss_filtering'] was turned on, the , ,  &
superglobals were automatically overwritten. This resulted in one of the following problems:

 - xss_clean() being called twice
 - Inability to retrieve the original (not filtered) value

XSS filtering is now only applied on demand by the Input class, and the default value for
the  parameter in CI_Input methods is changed to NULL. Unless a boolean value is
passed to them, whether XSS filtering is applied depends on the ['global_xss_filtering']
value.
diff --git a/system/helpers/cookie_helper.php b/system/helpers/cookie_helper.php
index 5cdcdd1..a79083a 100644
--- a/system/helpers/cookie_helper.php
+++ b/system/helpers/cookie_helper.php
@@ -74,8 +74,9 @@
 	 * @param	bool
 	 * @return	mixed
 	 */
-	function get_cookie($index, $xss_clean = FALSE)
+	function get_cookie($index, $xss_clean = NULL)
 	{
+		is_bool($xss_clean) OR $xss_clean = (config_item('global_xss_filtering') === TRUE);
 		$prefix = isset($_COOKIE[$index]) ? '' : config_item('cookie_prefix');
 		return get_instance()->input->cookie($prefix.$index, $xss_clean);
 	}