title: Nested form deletion not working Rails 3
body: <p>I have a project model, and a project can have many pictures.  The uploading and adding of the multiple photos is working fine.  however on my edit form I have checkboxes for removing whatever pictures are checked, and the subsequent form submision isn't deleting the record.</p>
  
  <p>Here is the output from the ruby debugger</p>
  
  <pre><code>(rdb:2) @project.latestprojectpics.detect {|p| p.id==3}.marked_for_destruction?
  true
  (rdb:2) @project.latestprojectpics.length
  1
  (rdb:2) @project.save
  true
  (rdb:2) @project.latestprojectpics.length
  1 
  </code></pre>
  
  <p>As you can see, the record is marked for destruction, but when saved it doesn't delete.</p>
  
  <p>I have allow_destroy=>true in the model.  And my Delete checkbox is called _destroy.  Everything seems to be wired up correctly.</p>
  
  <p>Using Rails 3.0.4, Ruby 1.9.2, Carrierwave for file uploads.</p>