Add headers if not set.

[SamMousa]

Currently this module adds support for a combined set/add which overwrites or adds a header, meaning the value of the header is always changed.

It would be great if it could also offer a set/skips function which sets if not present and skips otherwise.

[agentzh]

@SamMousa Patches welcome. BTW, such requirements can be easily and efficiently done via the ngx_http_lua module:

https://github.com/openresty/lua-nginx-module/

That's why I'm reluctant to develop more and more such little things myself in this simple module.

[mkasimd]

Another way to achieve that might be through a map, e.g. something like this:

map $sent_http_referrer_policy $referrer_policy {
    ""         "strict-origin-when-cross-origin";
    default    $sent_http_referrer_policy;
}
....
more_set_headers "Referrer-Policy: $referrer_policy";

Certainly, a short more_set_headers -i as in #65 offers significanlty better maintainability and readability, but the above solution should work too.