padrino-framework: Misbehavior of Slim, content_tag and html_safe

Here’s a strange test case for Padrino console

include Padrino::Helpers::OutputHelpers
include Padrino::Helpers::TagHelpers

> Slim::Template.new{ "= content_tag( :div, content_tag(:i) )" }.render
=> "<div><i></i></div>"
> Slim::Template.new{ "= content_tag( :div ) do\n  = content_tag(:i)" }.render
=> "<div>&lt;i&gt;&lt;/i&gt;</div>"

Is it expected behavior or failed processing of block in content_tag?