How to write a filter to nanoc that adds a ref to a toc after each header

I'm trying to create a filter that adds references to the TOC after each h2 heading.

Here is what I've done but it doesn't work, and each time I modify the code all the relevant pages are not recompiled.

{Project dir}/Rules:

compile '/**/*.md' do
  filter :kramdown
  layout '/default.*'
  filter :add_ref_to_toc
  filter :add_toc_french
  write item.identifier.without_ext + '/index.html'
end

{Project dir}/lib/filters/add_ref_to_toc.rb:

require 'nokogiri'
class AddRefToTocFilter < Nanoc::Filter

  identifier :add_ref_to_toc

  def run(content, params={})    
    doc = Nokogiri::HTML.fragment(content)
    doc.css('#contents h2') do |header|
      header.add_next_sibling "<br/><a href='#toccontent'>Aller à la table des matières</a><br/><br/>"
    end
    doc.to_s
  end

end

Page after processing:

<div id="contents">
<h2>title 1</h2>
<br/><a href='#toccontent'>Aller à la table des matières</a><br/><br/>

<h2>title 2</h2>
<br/><a href='#toccontent'>Aller à la table des matières</a><br/><br/>

<h2>title 3</h2>
<br/><a href='#toccontent'>Aller à la table des matières</a><br/><br/>
</div>