Added missing 'using'

IotaCSharpApi/Api/Utils/Rest/JsonWebClient.cs

@@ -33,7 +33,7 @@ public TResponse GetPOSTResponseSync<TResponse>(Uri uri, string data)
 
             request.ContentLength = bytes.Length;
 
-            Stream requestStream = request.GetRequestStream();
+            using (Stream requestStream = request.GetRequestStream())
             {
                 // Send the data.
                 requestStream.Write(bytes, 0, bytes.Length);