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.
I don't see where `endpoint` is an instance of `Grape::Endpoint`. In fact, I'm not seeing where `endpoints` is defined, either as a method or an variable. Am I just missing it?