Little improvements

Hello!

Thanks for the plugin! I want to suggest a few improvements:

1. Add header('Cache-Control: no-store, no-cache, must-revalidate'); before redirect header. Sometimes browsers may cache redirect url. In this case changes to “Reddirect to URL” will not work as expected for all users!

2. On line 55 when you check if(!is_admin()), add is_singular( 'qrcode' ) condition. That will ensure we are actually accessing redirect page! Just checking for admin and post_type is not enough. There is a possibility of infinite redirects. I got this issue on front page with all published posts in the loop!..