Fix a CI_Input::set_cookie() bug

Found on StackOverflow: https://stackoverflow.com/questions/41925028/codeigniter-config-overrides-set-cookie-parameters
diff --git a/system/helpers/cookie_helper.php b/system/helpers/cookie_helper.php
index bb90cba..b943edb 100644
--- a/system/helpers/cookie_helper.php
+++ b/system/helpers/cookie_helper.php
@@ -67,7 +67,7 @@
 	 * @param	bool	true makes the cookie accessible via http(s) only (no javascript)
 	 * @return	void
 	 */
-	function set_cookie($name, $value = '', $expire = '', $domain = '', $path = '/', $prefix = '', $secure = FALSE, $httponly = FALSE)
+	function set_cookie($name, $value = '', $expire = '', $domain = '', $path = '/', $prefix = '', $secure = NULL, $httponly = NULL)
 	{
 		// Set the config file options
 		get_instance()->input->set_cookie($name, $value, $expire, $domain, $path, $prefix, $secure, $httponly);