title: Rails ActiveAdmin Layout was lost after overriding Controller action
body: <p>Overriding a new controller without 'new!' does not display ActiveAdmin layout.  But then when I added 'new!' nested 'synchronization' form is not appearing although I did '@resource.build_synchronization'.  Not so sure what I'm doing wrong here.</p>
  
  <p>case #1 (ActiveAdmin layout is gone)</p>
  
  <pre><code>ActiveAdmin.register Resource do
    controller do
        # This code is evaluated within the controller class
        def new
          @resource = Resource.new
          @resource.build_synchronization
        end
    end
  end
  </code></pre>
  
  <p>case #2 (nested form synchronization does not appear)</p>
  
  <pre><code>ActiveAdmin.register Resource do
    controller do
        # This code is evaluated within the controller class
        def new
          @resource = Resource.new
          @resource.build_synchronization
          new!
  
        end
    end
  end
  </code></pre>
  
  <hr>
  
  <p>views\admin\resources\new.html.erb</p>
  
  <pre><code>&lt;%= semantic_form_for [:admin, @resource] do |form| %&gt;
      &lt;%= form.inputs "Resource", :id =&gt; "resource" do %&gt;
          &lt;%= form.input :name %&gt;
          &lt;%= form.semantic_fields_for :synchronization do |sync| %&gt;
              &lt;% sync.inputs  :name =&gt; "Synchronization", :id =&gt; "synchronization"  do %&gt;
                  &lt;%= sync.input :start_datetime, :as =&gt; :datetime %&gt;
                  &lt;%= sync.input :repeat_interval, :as =&gt; :radio, :collection =&gt; @intervals %&gt;
                  &lt;%= sync.input :repeat_type, :as =&gt; :select, :collection =&gt; ["Manual", "Automatic"] %&gt;
              &lt;% end %&gt;
          &lt;% end %&gt;
      &lt;% end %&gt;
      &lt;%= form.buttons %&gt;
  &lt;% end %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <hr>
  
  <p>models:</p>
  
  <pre><code>class Resource &lt; ActiveRecord::Base
    has_one :synchronization
    accepts_nested_attributes_for :synchronization
  end
  
  
  class Synchronization &lt; ActiveRecord::Base
    belongs_to :resource
    has_many :mappings
    accepts_nested_attributes_for :mappings
    #validates_presence_of :start_datetime
  end
  </code></pre>