Fixes the `target_class` option to fully work. The code structure here is highly questionable (no separation of concerns, for one thing). But the whole class needs some serious refactoring, so this will work until that happens, at least.
The README states that the path mounted by default will be swagger_doc.json but it's swagger_doc according to: https://github.com/tim-vandecasteele/grape-swagger/blob/master/lib/grape-swagger.rb#L470.
…includes README update.
…itly declared in the resource, e.g., `format :json`. TODO: Add spec(s).