Letters with diacritics (accents) are completely removed in TOC IDs

[DannyBen]

Originally posted on https://github.com/DannyBen/madness/issues/158

Reproduction Code

require 'redcarpet'

text = '## 2.4. Úprava plastové krabičky pro elektroniku'
renderer = Redcarpet::Render::HTML.new with_toc_data: true
html = Redcarpet::Markdown.new(renderer).render text
puts html

Actual Output

<h2 id="2-4-prava-plastov-krabi-ky-pro-elektroniku">
  2.4. Úprava plastové krabičky pro elektroniku
</h2>

Expected Output

<h2 id="2-4-úprava-plastové-krabičky-pro-elektroniku">
  2.4. Úprava plastové krabičky pro elektroniku
</h2>