Fix ManagedMqttClient missing call to unsubscribe topics

Frameworks/MQTTnet.NetStandard/ManagedClient/ManagedMqttClient.cs

@@ -16,6 +16,7 @@ namespace MQTTnet.ManagedClient
         private readonly BlockingCollection<MqttApplicationMessage> _messageQueue = new BlockingCollection<MqttApplicationMessage>();
         private readonly Dictionary<string, MqttQualityOfServiceLevel> _subscriptions = new Dictionary<string, MqttQualityOfServiceLevel>();
         private readonly SemaphoreSlim _subscriptionsSemaphore = new SemaphoreSlim(1, 1);
+        private readonly List<string> _unsubscriptions = new List<string>();
 
         private readonly IMqttClient _mqttClient;
         private readonly IMqttNetLogger _logger;
@@ -129,6 +130,7 @@ namespace MQTTnet.ManagedClient
                 {
                     if (_subscriptions.Remove(topic))
                     {
+                        _unsubscriptions.Add(topic);
                         _subscriptionsNotPushed = true;
                     }
                 }
@@ -274,16 +276,26 @@ namespace MQTTnet.ManagedClient
             _logger.Info<ManagedMqttClient>(nameof(ManagedMqttClient), "Synchronizing subscriptions");
 
             List<TopicFilter> subscriptions;
+            List<string> unsubscriptions;
+
             await _subscriptionsSemaphore.WaitAsync().ConfigureAwait(false);
             try
             {
                 subscriptions = _subscriptions.Select(i => new TopicFilter(i.Key, i.Value)).ToList();
+
+                unsubscriptions = new List<string>(_unsubscriptions);
+                _unsubscriptions.Clear();
+
                 _subscriptionsNotPushed = false;
             }
             finally
             {
                 _subscriptionsSemaphore.Release();
             }
+            if (unsubscriptions.Any())
+            {
+                await _mqttClient.UnsubscribeAsync(unsubscriptions);
+            }
 
             if (!subscriptions.Any())
             {