Modify PAC request behavior

Update the PAC file using the proxy channel only when proxy mode is enabled.

shadowsocks-csharp/Controller/Service/GfwListUpdater.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Net;
@@ -91,7 +91,10 @@ namespace Shadowsocks.Controller
         {
             Logging.Info($"Checking GFWList from {GFWLIST_URL}");
             WebClient http = new WebClient();
-            http.Proxy = new WebProxy(IPAddress.Loopback.ToString(), config.localPort);
+            if (config.enabled)
+            {
+                http.Proxy = new WebProxy(IPAddress.Loopback.ToString(), config.localPort);
+            }
             http.DownloadStringCompleted += http_DownloadStringCompleted;
             http.DownloadStringAsync(new Uri(GFWLIST_URL));
         }