Update proxy_host.conf

FIX as explained on #340

backend/templates/proxy_host.conf

@@ -32,7 +32,7 @@ server {
 
     {% if allow_websocket_upgrade == 1 or allow_websocket_upgrade == true %}
     proxy_set_header Upgrade $http_upgrade;
-    proxy_set_header Connection "upgrade";
+    proxy_set_header Connection $http_connection;
     proxy_http_version 1.1;
     {% endif %}