Added proper `as_json` support for `Results::Collection` and `Results::Item` classes

Previously, the `Collection` and `Item` classes were missing proper `as_json`
serialization support. With this patch, their `as_json` methods are now
ActiveSupport compatible, allowing to eg. expose the results in a Rails controller:

    def search
      @articles = Article.search do ... end
      respond_to do |format|
        format.html { render :action => "index" }
        format.json { render json: @articles.as_json(only: ['id', 'title']) }
      end
    end

Closes #527