How to redirect with Iron-Router without breaking Browser's back function

IronRouterRedirect.js

    // My Iron-Router map has these two routes
    // I'd like to redirect all requests to /forum/:id 
    // onward to /forum/:id/:slug
    // Using this.redirect(...) works but prevents the 
    // browser from navigating "back"
    
    this.route('questionDetailWithSlug', {
      template: 'questionDetail',
      path: '/forum/:id/:slug',
      action: function(){
        Session.set('questionId', this.params.id);
        this.render()
      },
      layoutTemplate: 'layout'
    });

    this.route('questionDetail', {
      path: '/forum/:id',
      action: function(){
        Session.set('questionId', this.params.id);
        var question = ForumQuestions.findOne(this.params.id);
        console.log(question);
        this.redirect("questionDetailWithSlug", {id: this.params.id, slug: question.slug})
      },
      layoutTemplate: 'layout'
    });