Preserving object references doesn't work

[Horusiath]

While working on #82 I've found out that SerializerOptions.PreserveObjectReferences actually never worked for user-defined objects. Example:

var serializer = new Serializer(new SerializerOptions(preserveObjectReferences: true));
var serializerSession = new SerializerSession(serializer);
var deserializerSession = new DeserializerSession(serializer);

using (var stream = new MemoryStream())
{
    var o = new MyClass {First = "hello", Second = 123};

    serializer.Serialize(o, stream, serializerSession); // serialize object 1st time
    stream.Position = 0;
    serializer.Serialize(o, stream, serializerSession); // serialize it again using the same session
}

First serialization passes, but second one throws ArgumentException: An item with the same key has already been added.. From what I've seen, generated object serializer simply doesn't look it object has been saved previously before trying to serialize it again.