Cover missed lines in ImageController#show_selected_images
Cover missed lines in ImageController#show_selected_images

ll. 167-177
```ruby
    # Paginate by letter if sorting by user.
    case query.params[:by]
    when "user", "reverse_user"
      args[:letters] = "users.login"
    # Paginate by letter if sorting by copyright holder.
    when "copyright_holder", "reverse_copyright_holder"
      args[:letters] = "images.copyright_holder"
    # Paginate by letter if sorting by name.
    when "name", "reverse_name"
      args[:letters] = "names.sort_name"
    end
```

MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer,MushroomObserver/mushroom-observer