Fixed a bug with + in query

shadowsocks-csharp/Model/Server.cs

@@ -169,7 +169,7 @@ namespace Shadowsocks.Model
                     server.password = userInfoParts[1];
 
                     NameValueCollection queryParameters = HttpUtility.ParseQueryString(parsedUrl.Query);
-                    string[] pluginParts = HttpUtility.UrlDecode(queryParameters["plugin"] ?? "").Split(new[] { ';' }, 2);
+                    string[] pluginParts = (queryParameters["plugin"] ?? "").Split(new[] { ';' }, 2);
                     if (pluginParts.Length > 0)
                     {
                         server.plugin = pluginParts[0] ?? "";