ActionView check for NilClass & unextended models

I've added some checks for NilClass and Unextended models in an empty Array (missing methods: entry_name, limit_value, offset_value, total_count), which are used in `lib/kaminari/helpers/action_view_extension.rb` in helper methods: paginate, link_to_previous_page, link_to_next_page, page_entries_info, and rel_next_prev_link_tags.

This way I can safely use those helpers, without having to manually check their types in the views (reducing the chance of accidental issues). Below a sample code snippet: 

```haml
# controller
unless current_user.nil?
  @posts = current_user.all_posts
end
# view
div.paginate
  = paginate @posts
  = link_to_previous_page @posts, "<< Previous"
  = link_to_next_page @posts, "Next >>"
  = page_entries_info @posts
  = rel_next_prev_link_tags @posts
```

PS. I've included tests for these altered helpers as well. All `action_view_extension.rb` tests are still successful.