VertexCollection will NOT build correct documents

Ok, this is some kind of major FUBAR… Somehow it was forgotten to implement the `build_content_class` method in `VertexCollection`. The one to blame is definitely me. I'm very sorry. Additionally the `Document.new` in `Collection#create_document` must be replaced with `build_content_class`.

**This is a major bug and should be fixed immediately**

Since this bug can lead to data corruption we should discuss how we want to communicate it and handle the current released version.