title: ActionController::UnknownFormat when I use respond_to + respond_with (Rails 4)
body: <p>I can't understand what's happening, when I ask for index action I receive ActionController::UnknownFormat, even existing a index.js file inside views/products folder. Anyone can help me to figure out what I'm missing out? I'm using Rails 4 and Ruby 2.0.0-p247.</p>
  
  <pre><code>class ProductsController &lt; ApplicationController
    respond_to :js
    def index
      @products = Product.published.newest.page(params[:page])
      respond_with @products
    end
  
  end
  </code></pre>