Fixed the issue where nil for header_ids

[ppworks]

Summary

I have fixed the issue where specifying nil for :header_ids in the :extension config would result in the nil option being cleared when reusing the config.

Target Version

https://github.com/gjtorikian/commonmarker/tree/v2.0.0/ext/commonmarker

Use Case

user_config = {
  extension: {
    header_ids: nil,
  },
}

text = "# Heading-1"

doc = Commonmarker.parse(text, options: user_config) # In this process, header_ids is nil, so it gets removed. 
doc.walk do |node|
  # do something
end

puts doc.to_html(options: user_config)

Expected Output

<h1>Heading-1</h1>

Actual Output in

<h1><a href=\"#heading-1\" aria-hidden=\"true\" class=\"anchor\" id=\"heading-1\"></a>Heading-1</h1>

[gjtorikian]

Thank you! ✨