Be able to set headers with always option set to true

[aslafy-z]

Add the ability to use ngx.req.set_header with the always option set to true.

See:

• http://nginx.org/en/docs/http/ngx_http_headers_module.html#add_header
• https://github.com/nginx/nginx/blob/master/src/http/modules/ngx_http_headers_filter_module.c#L858

I haven't found any reference to it. As I know, it would have to be implemented in here: https://github.com/openresty/lua-nginx-module/blob/master/src/ngx_http_lua_headers_in.c#L180

I'd be glad to work this on but I don't really know where to start. I found that the structures where not the same between lua-nginx-module and nginx itself so if someone can help me kicking in, i'm gona take the issue.

Pending: Kong/kong#1424

[leandromoreira]

I'm also interested in this