Improve RPC client handling.

Source/MQTTnet.Extensions.Rpc/MqttRpcClientExtensions.cs

@@ -0,0 +1,19 @@
+using System;
+using System.Text;
+using System.Threading.Tasks;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Extensions.Rpc
+{
+    public static class MqttRpcClientExtensions
+    {
+        public static Task<byte[]> ExecuteAsync(this IMqttRpcClient client, TimeSpan timeout, string methodName, string payload, MqttQualityOfServiceLevel qualityOfServiceLevel)
+        {
+            if (client == null) throw new ArgumentNullException(nameof(client));
+
+            var buffer = Encoding.UTF8.GetBytes(payload ?? string.Empty);
+
+            return client.ExecuteAsync(timeout, methodName, buffer, qualityOfServiceLevel);
+        }
+    }
+}