title: ActiveAdmin throws undefined method `stringify_keys'
body: <p>I am Using ActiveAdmin 0.4.1 with Rails 3.2.1. 
  In a plain rails app I create a new model via scaffolding and set it up as a resource for ActiveAdmin. When I try to create a new entity of my mode using the ActiveAdmin backend I get</p>
  
  <pre><code>undefined method `stringify_keys' for "new":String
  </code></pre>
  
  <p>without any application traces. Only the framework trace:</p>
  
  <pre><code>activerecord (3.2.1) lib/active_record/attribute_assignment.rb:69:in `assign_attributes'
  activerecord (3.2.1) lib/active_record/base.rb:495:in `initialize'
  inherited_resources (1.3.0) lib/inherited_resources/base_helpers.rb:52:in `new'
  inherited_resources (1.3.0) lib/inherited_resources/base_helpers.rb:52:in `build_resource'
  activeadmin (0.4.1) lib/active_admin/resource_controller/callbacks.rb:15:in `build_resource'
  - snip -
  </code></pre>
  
  <p>What could be a possible solution to this?</p>