SocketIO.Serializer.MessagePack nuget package is missing SerializeUpgradeMessage #397
Description
Currently published latest (3.1.2) MessagePack serializer version is missing SerializeUpgradeMessage (https://github.com/doghappy/socket.io-client-csharp/blob/master/src/SocketIO.Serializer.MessagePack/SocketIOMessagePackSerializer.cs#L268) from ISerializer, and therefore fails to load.
