fixed tests and added testcase for user property serialization

Source/MQTTnet.AspnetCore/SpanBasedMqttPacketBodyReader.cs

@@ -51,6 +51,11 @@ namespace MQTTnet.AspNetCore
         public unsafe string ReadStringWithLengthPrefix()
         {
             var buffer = ReadSegmentWithLengthPrefix();
+            if (buffer.Length == 0)
+            {
+                return string.Empty;
+            }
+
             fixed (byte* bytes = &buffer.GetPinnableReference())
             {
                 var result = Encoding.UTF8.GetString(bytes, buffer.Length);

Source/MQTTnet.AspnetCore/SpanBasedMqttPacketWriter.cs

@@ -60,12 +60,11 @@ namespace MQTTnet.AspNetCore
 
         public void Write(IMqttPacketWriter propertyWriter)
         {
-            if (propertyWriter is SpanBasedMqttPacketWriter writer)
-            {
-                GrowIfNeeded(1);
-            }
+            if (propertyWriter == null) throw new ArgumentNullException(nameof(propertyWriter));
 
-            throw new InvalidOperationException($"{nameof(propertyWriter)} must be of type {typeof(SpanBasedMqttPacketWriter).Name}");
+            GrowIfNeeded(propertyWriter.Length);
+            Write(propertyWriter.GetBuffer(), 0, propertyWriter.Length);
+            Commit(propertyWriter.Length);
         }
 
         public void Write(byte[] payload, int start, int length)

Source/MQTTnet/Formatter/MqttPacketFormatterAdapter.cs

@@ -89,28 +89,30 @@ namespace MQTTnet.Formatter
             }
 
             ProtocolVersion = protocolVersion;
+            _formatter = GetMqttPacketFormatter(protocolVersion, Writer);
+        }
 
+        public static IMqttPacketFormatter GetMqttPacketFormatter(MqttProtocolVersion protocolVersion, IMqttPacketWriter writer)
+        {
+            if (protocolVersion == MqttProtocolVersion.Unknown)
+            {
+                throw new InvalidOperationException("MQTT protocol version is invalid.");
+            }
+            
             switch (protocolVersion)
             {
                 case MqttProtocolVersion.V500:
                     {
-                        _formatter = new MqttV500PacketFormatter(Writer);
-                        break;
+                        return new MqttV500PacketFormatter(writer);
                     }
-
                 case MqttProtocolVersion.V311:
                     {
-                        _formatter = new MqttV311PacketFormatter(Writer);
-                        break;
+                        return new MqttV311PacketFormatter(writer);
                     }
-
                 case MqttProtocolVersion.V310:
                     {
-
-                        _formatter = new MqttV310PacketFormatter(Writer);
-                        break;
+                        return new MqttV310PacketFormatter(writer);
                     }
-                    
                 default:
                     {
                         throw new NotSupportedException();

Source/MQTTnet/Formatter/MqttPacketWriter.cs

@@ -154,6 +154,7 @@ namespace MQTTnet.Formatter
                 }
 
                 Write(writer._buffer, 0, writer.Length);
+                return;
             }
 
             throw new InvalidOperationException($"{nameof(propertyWriter)} must be of type {typeof(MqttPacketWriter).Name}");