duplicate-headers: prevent some headers from being copied over

Amongst other headers, don't copy the "Connection" response-header
when we receive it from psol.

Combined with our config trick[1] to prevent add_header from setting
any other headers should fix 
https://github.com/pagespeed/ngx_pagespeed/issues/314 (yet to be confirmed)

[1] location ~ "\.pagespeed\.([a-z]\.)?[a-z]{2}\.[^.]{10}\.[^.]+" {
      add_header "" "";
    }