"msie_padding on" doesn't work?

[Lorenzo Raffio]

msie_padding is set to on by default (and I didn't change it). Today I
added this line to my location php block:
    if ($http_user_agent ~ "MSIE 6" ) {
      return 403 "Browser not supported. Please update or change to another
one.";
    }

So now it is:
location ~ \.php$ {
    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    include /etc/nginx/fastcgi.conf;
    fastcgi_pass unix:/var/run/php5-fpm.sock;
    if ($http_user_agent ~ "MSIE 6" ) {
      return 403 "Browser not supported. Please update or change to another
one.";
    }
}

It works (with curl and a IE6 user-agent I get my custom error message),
the problem is that IE6 (real browser) displays its default error page
(because my message is < of 512 bytes). But msie_padding on should "fix"
that, am I wrong?

If I change the error to something longer it works! For example I set one
(in italian, sorry): "Internet Explorer 6.0 non e' supportato. Per poter
visualizzare il sito aggiorna ad una versione successiva o installa un
browser alternativo (Google Chrome, Mozilla Firefox, Opera ...)." and it
displays my error.
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