title: How can I use image_path inside Rails 3 Controller
body: <p>An identical question has been asked before 
  <a href="https://stackoverflow.com/questions/3804082/assettaghelperimage-path-outside-views">AssetTagHelper::image_path outside views</a> 
  but the solution does not work for Rails 3. </p>
  
  <p>I have a requirement where I am returning some data using JSON format and part of the data needs to return the full path of the image. I am building the response in the controller and calling render :json. I am also specifying an asset_host in the environment.rb so I need a way to include that in the returned data from inside a Rails controller. </p>