REXML raw tag not working when getting an attribute?

Today I met a problem and want REXML get the raw text from attibute, e.g.

    xml_string="<root><outline url='&amp;'/></root>"
    doc= REXML::Document.new( xml_string, {:raw => :all} )
    outline = doc.root.elements[1]
    url = outline.attribute("url")  # raw text!
    url = outline.attributes["url"]  # escaped text!

solution is:  use  attribute("xx") instead of attributes["xx"]. 

p.s. How I want to use LibXML and say goodbye to REXML....