GraphMLSerializer does not serialize derived vertex/edge attributes

[leryss]

Seems the serializer just looks at the base type of the vertex for attribute serialization. Is this by design ?

https://github.com/YaccConstructor/QuickGraph/blob/50e0e16df0e6923d7a91e0fbabc89adee451145c/src/QuickGraph/Serialization/GraphMLSerializer.cs#L452-L455