title: mongoid: update nested attributes
body: <p>i am running into an issue with updates to nested attributes for referenced documents not being persisted</p>
  
  <pre><code>&gt; u = User.first
   =&gt; #&lt;User...
  &gt; u.changes
   =&gt; {} 
  &gt; u.profile.changes
   =&gt; {} 
  &gt; u.attributes = {:profile_attributes =&gt; {:weight =&gt; 8}}
   =&gt; {:profile_attributes=&gt;{:weight=&gt;8}} 
  &gt; u.changes
   =&gt; {} 
  &gt; u.profile.changes
   =&gt; {"weight"=&gt;[14.0, 8.0]} 
  &gt; u.update
   =&gt; true 
  &gt; u.shipping_profile.changes
   =&gt; {"weight"=&gt;[14.0, 8.0]}
  </code></pre>
  
  <p>update/update_attributes/save all do not persist the nested changes.
  i have <code>attr_accessible :profile_attributes</code> declared in the user model.</p>
  
  <p>what is needed for nested attributes to persist when the parent document is already persisted?</p>