use underscore when appending controller to the default controller

string. calling camelize on a namespaced controller will result in
Api::Mycontroller instead of Api::MyController

Author: kevinmarx

lib/apipie/application.rb

@@ -55,7 +55,7 @@ def rails_routes(route_set = nil)
     # this method does in depth search for the route controller
     def route_app_controller(app, route, visited_apps = [])
       if route.defaults[:controller]
-        controller_name = (route.defaults[:controller] + 'Controller').camelize
+        controller_name = "#{route.defaults[:controller]}_controller".camelize
         controller_name.safe_constantize
       end
     end