options[:format] = :xhtml not working

[back2war]

Hello, I'm trying to get the same behavior for :xhtml option as in Haml 5, but it doesn't work.

config/initializers/haml.rb
Haml::Template.options[:format] = :xhtml

app/views/test/index.html.haml
%p{ selected: true, data: { boolean_true: true, boolean: false } }

Result:

https://github.com/back2war/haml6-test

Could you please advise how to get XHTML like attributes?