title: add/delete items from Ember Data backed ArrayController
body: <p>I'm using an ArrayController in my application that is fed from a Ember Data REST call via the application's Router:</p>
  
  <pre><code>postsController.connectOutlet('comment', App.Comment.find({post_id: post_id}));
  </code></pre>
  
  <p>For the Post UI, I have the ability to add/remove Comments.   When I do this, I'd like to be able to update the contentArray of the postsController by deleting or adding the same element to give the user visual feedback, but Ember Data is no fun:</p>
  
  <pre><code>Uncaught Error: The result of a server query (on App.Comment) is immutable.
  </code></pre>
  
  <p>Per sly7_7's comment below, I just noticed that the result is indeed DS.RecordArray when there is no query (App.Comment.find()), but in the case where there is a query (App.Comment.find({post_id: post_id}), a DS.AdapterPopulatedRecordArray is returned.</p>
  
  <p>Do I have to .observes('contentArray') and create a mutable copy?  Or is there a better way of doing this?</p>