In any case, I just took a quick look through index.php again. Correct me if I am wrong but, it looks like the only place it outputs a header with Content-type is where there has been an issue with the backend not being able to handle the request.
In this case, the entire error page is generated from the index.php script. So it hardly matters if it is utf-8 encoded or not.
If the request has been handled, then the Content-type should already have come from the backend.