Fixed an TLS issue in the MqttClientOptionsBuilder; Fix a subscription issue in the managed client.

Frameworks/MQTTnet.NetStandard/Client/MqttClient.cs

@@ -12,7 +12,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Client
 {
-    public class MqttClient : IMqttClient
+    public class MqttClient : IMqttClient, IDisposable
     {
         private readonly IMqttClientAdapterFactory _adapterFactory;
         private readonly MqttPacketDispatcher _packetDispatcher;
@@ -180,6 +180,11 @@ namespace MQTTnet.Client
             }
         }
 
+        public void Dispose()
+        {
+            _cancellationTokenSource?.Dispose();
+        }
+
         private async Task<MqttConnAckPacket> AuthenticateAsync(MqttApplicationMessage willApplicationMessage)
         {
             var connectPacket = new MqttConnectPacket