Support namespaces in Grape

[tombruijn]

Only the path was used which does not include the namespace. This is
only available on the Grape::Endpoint.

Namespaces and paths can be anything, Symbols or Strings, prefixed with
a / or not.

Brain dump about thoughts on the integration and tests:

I kept the fallback on the request data the same right now, but I think we can take that out at some point. Same as with the Sinatra integration. If env["api.endpoint"] doesn't exist the integration provides poorly structured data and it's of not much use. We should perform some tests later to see if there can ever not be api.endpoint in the env. Also see if we can take a different testing approach, something a bit more full stack approach with Rake::Test. Now we control how the middleware is created, something which we don't in grape.

As reported in https://app.intercom.io/a/apps/yzor8gyw/inbox/conversation/5431615974