redirect_trailing_slash doesn't work with custom_base_url

[ViliusS]

redirect_trailing_slash funcionality doesn't respect custom_base_url path. For example https://my.example.com/custombaseurl/page1/ will be redirected to https://my.example.com/page1

It looks like $url in Location: redirect here doesn't include custom_base_url at all:

grav/system/src/Grav/Common/Grav.php

Line 500 in ee8d783

return new Response($code, ['Location' => $url]);

Sadly, I'm not sure what is the best way to fix this.