only downcase ascii characters on ruby 2.4+

Starting in Ruby 2.4, the String case conversion methods are Unicode
aware. To ensure that anchors in TOC links remain stable when upgrading
to Ruby 2.4, we need to preserve the old behaviour by passing :ascii
to String#downcase

Author: Charlie Somerville

lib/html/pipeline/toc_filter.rb

@@ -36,7 +36,7 @@ def call
         headers = Hash.new(0)
         doc.css('h1, h2, h3, h4, h5, h6').each do |node|
           text = node.text
-          id = text.downcase
+          id = ascii_downcase(text)
           id.gsub!(PUNCTUATION_REGEXP, '') # remove punctuation
           id.gsub!(' ', '-') # replace spaces with dash
 
@@ -50,6 +50,16 @@ def call
         result[:toc] = %Q{<ul class="section-nav">\n#{result[:toc]}</ul>} unless result[:toc].empty?
         doc
       end
+
+      if RUBY_VERSION >= "2.4"
+        def ascii_downcase(str)
+          str.downcase(:ascii)
+        end
+      else
+        def ascii_downcase(str)
+          str.downcase
+        end
+      end
     end
   end
 end