Fixed fetching namespace options for mounted APIs.

This fixes the scenario where an API is mounted by another API, in the simple case. 

There's a bigger issue is that namespaces can nest and that swagger namespaces != grape namespaces, but that's another can of worms, https://github.com/tim-vandecasteele/grape-swagger/issues/130.
This is an extension to the [Grape class](https://github.com/intridea/grape/blob/master/lib/grape/endpoint.rb#L382).