Display correct pagination figures when @per_page is set

[DMajrekar]

Setting a custom @per_page through a before_filter fetches the correct number of records but the text in the pagination is incorrect.

Using collection.size rather than active_admin_application.default_per_page yields the correct text.

[pcreux]

This is great. We should add this as an option to the index method.

ActiveAdmin.register Category do
  index :per_page => 10 do
    # ...
  end
end