fixed tests

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketSerializer.cs

@@ -397,7 +397,7 @@ namespace MQTTnet.Serializer
                 throw new MqttProtocolViolationException("Protocol version not supported.");
             }
 
-            stream.Write((byte)packet.ConnectReturnCode);
+            stream.WriteByte((byte)packet.ConnectReturnCode);
 
             return MqttPacketWriter.BuildFixedHeader(MqttControlPacketType.ConnAck);
         }

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketWriter.cs

@@ -25,7 +25,7 @@ namespace MQTTnet.Serializer
         {
             if (value == null) throw new ArgumentNullException(nameof(value));
 
-            stream.Write(value.Value);
+            stream.WriteByte(value.Value);
         }
 
         public static void WriteWithLengthPrefix(this Stream stream, string value)