Allow calling root route on app

Fixes #64

Previously calling a root registered route would result in an error
message "Not Found" suggesting the route hadn't been registed, yet when
listing the routes, `fn routes list myapp` you could see the `/` route.

You can now successfully call a root registered route with `fn call
myapp /`

api/server/server.go

@@ -327,6 +327,7 @@ func (s *Server) bindHandlers(ctx context.Context) {
 
 	engine.DELETE("/tasks", s.handleTaskRequest)
 	engine.GET("/tasks", s.handleTaskRequest)
+	engine.Any("/r/:app", s.handleRunnerRequest)
 	engine.Any("/r/:app/*route", s.handleRunnerRequest)
 
 	// This final route is used for extensions, see Server.Add