Update docs.

Build/MQTTnet.nuspec

@@ -12,6 +12,7 @@
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
     <releaseNotes> * [Core] Performance optimizations.
 * [Core] Due to performance reasons the timestamp of log messages is now in UTC format.
+* [Core] Added several packet validations.
 * [Client] Received messages are now processed in the worker thread by default. Added a new setting for switching back to dedicated threads.
 * [ManagedClient] Fixed a loading issue of stored application messages (thanks to @JTrotta).
 * [Server] Added support for other WebSocket sub protocol formats like mqttv-3.1.1 (thanks to @israellot).

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketSerializer.cs

@@ -345,12 +345,12 @@ namespace MQTTnet.Serializer
             if (ProtocolVersion == MqttProtocolVersion.V311)
             {
                 stream.WriteWithLengthPrefix("MQTT");
-                stream.WriteByte(0x04); // 3.1.2.2 Protocol Level 4
+                stream.WriteByte(4); // 3.1.2.2 Protocol Level 4
             }
             else
             {
                 stream.WriteWithLengthPrefix("MQIsdp");
-                stream.WriteByte(0x03); // Protocol Level 3
+                stream.WriteByte(3); // Protocol Level 3
             }
 
             var connectFlags = new ByteWriter(); // 3.1.2.3 Connect Flags
@@ -410,6 +410,7 @@ namespace MQTTnet.Serializer
             {
                 var connectAcknowledgeFlags = new ByteWriter();
                 connectAcknowledgeFlags.Write(packet.IsSessionPresent);
+
                 stream.Write(connectAcknowledgeFlags);
             }
             else