fastcgi_cache_key and empty $content_type in cache

[namesys]

Have a strange behavior of fastcgi cache, the variable $content_type
does not passes to the cache KEY, configuration:
.....
location ~ \.php$ {
        fastcgi_cache cache;
        fastcgi_cache_key
"$content_type|$request_method|$is_args|$host|$request_uri|$http_referer";
        fastcgi_cache_valid any 60m;
        fastcgi_cache_use_stale error timeout invalid_header http_500
http_503;
        fastcgi_ignore_headers "Set-Cookie" "Cache-Control" "Expires";
        fastcgi_pass_header "Set-Cookie";

        fastcgi_pass   127.0.0.1:8000;
        fastcgi_index  index.php;
        fastcgi_connect_timeout 60;
        fastcgi_send_timeout 180;
        fastcgi_read_timeout 180;
        fastcgi_buffers 4 128k;                                         
     
        fastcgi_buffer_size 128k;                                       
     
        fastcgi_intercept_errors on;

        fastcgi_param  SCRIPT_FILENAME    
$document_root$fastcgi_script_name;
        fastcgi_param  REMOTE_USER           $remote_user;
        fastcgi_param  QUERY_STRING         $query_string;
        fastcgi_param  REQUEST_METHOD    $request_method;
        fastcgi_param  CONTENT_TYPE         $content_type;
        fastcgi_param  CONTENT_LENGTH     $content_length;
        fastcgi_param  SCRIPT_NAME            $fastcgi_script_name;
        fastcgi_param  REQUEST_URI            $request_uri;
        fastcgi_param  DOCUMENT_URI         $document_uri;
        fastcgi_param  DOCUMENT_ROOT      $document_root;
        fastcgi_param  SERVER_PROTOCOL    $server_protocol;
        fastcgi_param  GATEWAY_INTERFACE CGI/1.1;
        fastcgi_param  SERVER_SOFTWARE    nginx/$nginx_version;
        fastcgi_param  REMOTE_ADDR            $remote_addr;
        fastcgi_param  REMOTE_PORT            $remote_port;
        fastcgi_param  SERVER_ADDR             $server_addr;
        fastcgi_param  SERVER_PORT             $server_port;
        fastcgi_param  SERVER_NAME             $server_name;
      
        fastcgi_param SCRIPT_FILENAME
/var/www/site_folder$fastcgi_script_name;
}
.......

But when I look into cache-file, the KEY field does not contain the
content-type:
.......
KEY: 
|GET|?|site_name|/system/files/imagecache/50x50/iconimage.png|http://site_name/
.......
Content-Type: image/png
.......

As you see the content-type is visible in cache-file body, but it is not
visible in the KEY field, it's just empty there. Does anybody knows why
this happens?

Thank you for your advices.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,194424,194424#msg-194424