nixos/nginx: Do not remove headers while proxying

Removing the `Accept-Encoding` header breaks applications which may
produce already compressed content.

Removing this header is staded in the nginx docs but is ment as an
example, not as an recomendation.

(cherry picked from commit 762ca64)

nixos/modules/services/web-servers/nginx/default.nix

@@ -34,7 +34,6 @@ let
     proxy_set_header        X-Forwarded-Proto $scheme;
     proxy_set_header        X-Forwarded-Host $host;
     proxy_set_header        X-Forwarded-Server $host;
-    proxy_set_header        Accept-Encoding "";
   '';
 
   upstreamConfig = toString (flip mapAttrsToList cfg.upstreams (name: upstream: ''