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.
Because in this case it will always be an `Edge`. The `@content_class` is only useful in the generic `Collection` in my opinion. In the concrete collection classes we could just pin the document class.