Mounted nested API

My APIs look like [this](https://github.com/dblock/grape-swagger/blob/mounted-namespaced-api/spec/api_with_prefix_and_namespace_spec.rb). It has a prefix and a namespace.

I can't get that `status` endpoint documented. This returns:

```
[{"path"=>"/swagger_doc.{format}", "description"=>"Operations about swagger_docs"}]
```

Probably related to #220.
