collection_path throws missing key exception when using belongs_to

``` ruby
ActiveAdmin.register Comment do
 belongs_to :post
 csv_importable columns: [:name]
end
```

Calls to `collection_path` within the the `dsl.rb` will raise an exception.

`No route matches {:action=>"index", :controller=>"admin/comments", :post_id=>nil} missing required keys: [:post_id]`
