Fixed "private method render"-bug when trying to save invalid model under Rails 4.

Bug occurs when trying to save an invalid model under Rails 4.

Unfortunately I was not able to write a test for it, since I couldn't figure out how you do that in ActiveAdmin :-(

Be here is what I would be going for:

Alter Post-class to validate:
```ruby
class Post
  validates_presence_of :title
end
```

Controller spec:
```ruby
describe Admin::PostsController, type: :controller do
  render_views

  it "#create" do
    post :create, post: {title: ""}
    response.should render_template("new")
  end
end
```

Should not throw "private method render..."-error.