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

[1] location ~ ".pagespeed.([a-z].)?[a-z]{2}.[^.]{10}.[^.]+" {
add_header "" "";
}
Merge pull request #321 from pagespeed/oschaaf-duplicate-headers

duplicate-headers: prevent some headers from being copied over