nixos/firefox-syncserver: enable recommendedProxySettings

The syncserver requires a Host header identical to the one provided by
the client.

nixos/modules/services/networking/firefox-syncserver.nix

@@ -304,6 +304,10 @@ in
         forceSSL = cfg.singleNode.enableTLS;
         locations."/" = {
           proxyPass = "http://127.0.0.1:${toString cfg.settings.port}";
+          # We need to pass the Host header that matches the original Host header. Otherwise,
+          # Hawk authentication will fail (because it assumes that the client and server see
+          # the same value of the Host header).
+          recommendedProxySettings = true;
         };
       };
     };