Fix linking to ActiveAdmin actions for models that define to_param

If a model defines `to_param` in order to display pretty SEO-friendly URL's, ActiveAdmin links don't work.  For example:

```ruby
class Post < ActiveRecord::Base
  attr_accessible :title, :permalink
  
  before_create :generate_permalink
  
  def to_param
    permalink
  end

protected

  def generate_permalink
    permalink ||= title.parameterize
  end
end
```

In ActiveAdmin, all the links are called in a manner such as `admin_post_path(post)`, which will become `/admin/posts/hello-world` for example.  When ActiveAdmin attempts to load that page, RecordNotFound is raised since it's performing a find on the `id` column, not the `permalink` column.

The solution is to build all links with the record's `id`, e.g.: `admin_post_path(post.id)`.

This pull request implements the above fix.  It also includes an updated test that, if tested on the code prior to this pull request, will demonstrate the error.