Replaced ContainsKey and getting value by key with TryGetValue

(cherry picked from commit 084b6fe72c)

shadowsocks-csharp/Controller/I18N.cs

@@ -46,10 +46,7 @@ namespace Shadowsocks.Controller
 
         public static string GetString(string key, params object[] args)
         {
-            return _strings.ContainsKey(key)
-                ? string.Format(_strings[key], args)
-                : string.Format(key, args);
+            return string.Format(_strings.TryGetValue(key, out var value) ? value : key, args);
         }
-
     }
 }