Leading slash on 'X-Accel-Redirect' header

The header set by the endpoint finish callback to redirect to the newly processed image doesn't add a leading slash to the image path. 

This caused the redirect (when using a pretty typical nginx.conf) to be caught by the processing rule again, which just returns a 404 for that path instead of serving the new image. Refreshing the URL would then return the correct image. 

My nginx conf is:
```
http {
    sendfile on;

    upstream bobrosstwisted {
	    server 127.0.0.1:5500 max_fails=0;
    }
    server {
        listen       80;
        server_name  localhost;

        location / {
            root /srv/http/cache;
            rewrite ^/([0-9]+)/([0-9]+)/([a-z]+)/(.*)\.(.*)$ /$4_$1x$2_$3.$5 break;
            expires 1w;
            try_files $uri @bobross;
        }
        location @bobross {
            rewrite ^/(.*)\_([0-9]+)x([0-9]+)_([a-z]+)\.(.*)$ /$1.$5?width=$2&height=$3&mode=$4 break;
	    proxy_redirect off;
            proxy_buffering off;
            proxy_pass http://bobrosstwisted;
        }
        error_page   500 502 503 504  /50x.html;
        location = /50x.html {
            root   /srv/http;
        }
    }
}
events {}
```