title: Rabl json collection dynamic keys
body: <p>I have a User model that has many posts. Im trying to render the post in a rabl collection.</p>
  
  <pre><code>collection @posts
  attributes :title, :created_at
  </code></pre>
  
  <p>The default rabl format for rendering json collections is 
  <code>{'posts': ['post':{}, 'post':{}]}</code> . What I'm trying to accomplish is to change the 'post' key to a dynamic value post id for instance, so that my format would look like so <code>{'posts':[1:{}, 2:{} ]}</code>. Was trying with the configure of Rabl but the only thing I found out is how to get rid of the collection key with</p>
  
  <pre><code>Rabl.configure do |config|
    config.include_json_root = false
  end
  </code></pre>