Builder::XmlMarkup should escape "quotes" in attributes, right?

Railsers:

What's with Builder::XmlMarkup doesn't escape double quotes in tag
attributes? Ogle:

  def test_xml_markup
    x = Builder::XmlMarkup.new
    x.span :id => 'I be a "quoted" string'
    assert_not_equal '<span id="I be a "quoted" string"/>', x.target!
  end

That fails its assertion, and would also fail a check for well-formed XML.

I was relying on XmlMarkup to detect _how_ to mark quotes up, so I
wouldn't need to research it myself. &quot; is supported everywhere,
right? I'm going to need to get arbitrarily nested here (using the
XmlMarkup to build XHTML-style HTML that transports arbitrarily
complex strings), so I need perfectly balanced escapes and de-escapes
here.