Refactor serializer benchmark to allow fair comparisons.

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -192,7 +192,7 @@ namespace MQTTnet.Adapter
                     bodyOffset += readBytes;
                 } while (bodyOffset < body.Length);
 
-                return new ReceivedMqttPacket(fixedHeader.Flags, new MqttPacketBodyReader(body));
+                return new ReceivedMqttPacket(fixedHeader.Flags, new MqttPacketBodyReader(body, 0));
             }
             finally
             {

Source/MQTTnet/Serializer/MqttPacketBodyReader.cs

@@ -8,9 +8,10 @@ namespace MQTTnet.Serializer
         private readonly byte[] _buffer;
         private int _offset;
 
-        public MqttPacketBodyReader(byte[] buffer)
+        public MqttPacketBodyReader(byte[] buffer, int offset)
         {
             _buffer = buffer;
+            _offset = offset;
         }
 
         public int Length => _buffer.Length - _offset;