How to use nginx to proxy to a host requiring authentication?

For example:

 location / {
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_pass http://6.6.6.6:80;
    proxy_set_header Authorization "Basic a2luZzppc25ha2Vk";
 }

«a2luZzppc25ha2Vk» is «king:isnaked» base64 encoded, so that would work for