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.
Where do you see the advantage of:

```ruby
build_content_class(response, attributes)
```

```ruby
@content_class.new(@database, response, attributes)
```