Fix issues in RPC extension.

Frameworks/MQTTnet.NetStandard/MqttApplicationMessageBuilder.cs

@@ -32,7 +32,12 @@ namespace MQTTnet
             return this;
         }
 
-        public MqttApplicationMessageBuilder WithPayload(MemoryStream payload)
+        public MqttApplicationMessageBuilder WithPayload(Stream payload)
+        {
+            return WithPayload(payload, payload.Length - payload.Position);
+        }
+
+        public MqttApplicationMessageBuilder WithPayload(Stream payload, long length)
         {
             if (payload == null)
             {
@@ -46,7 +51,7 @@ namespace MQTTnet
             }
             else
             {
-                _payload = new byte[payload.Length - payload.Position];
+                _payload = new byte[length];
                 payload.Read(_payload, 0, _payload.Length);
             }
 

Frameworks/MQTTnet.NetStandard/Packets/MqttUnsubscribe.cs

@@ -11,7 +11,7 @@ namespace MQTTnet.Packets
         public override string ToString()
         {
             var topicFiltersText = string.Join(",", TopicFilters);
-            return "Subscribe: [PacketIdentifier=" + PacketIdentifier + "] [TopicFilters=" + topicFiltersText + "]";
+            return "Unsubscribe: [PacketIdentifier=" + PacketIdentifier + "] [TopicFilters=" + topicFiltersText + "]";
         }
     }
 }