airsonic: enable nginx.recommendedProxySettings with virtualHost

This fixes music playback when using the `services.airsonic.virtualHost`
option.

nixos/modules/services/misc/airsonic.nix

@@ -138,6 +138,7 @@ in {
 
     services.nginx = mkIf (cfg.virtualHost != null) {
       enable = true;
+      recommendedProxySettings = true;
       virtualHosts.${cfg.virtualHost} = {
         locations.${cfg.contextPath}.proxyPass = "http://${cfg.listenAddress}:${toString cfg.port}";
       };