Rewrite: Add additional `proxy_set_header`s to strip

Adding `proxy_set_header Accept-Encoding` is not enough because it overrides the other `proxy_set_header`s. From http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_set_header it says, "These directives are inherited from the previous level if and only if there are no proxy_set_header directives defined on the current level". 

Adding a file that contains internal `proxy_set_header`s (which strips `User-Agent`, `Cookies`, etc.) fixes this.