def diff(nocolor = false)
      xml = Nokogiri::XML(File.open(@path), &:noblanks)
      ideal = xml.to_xml(indent: 2)
      now = File.read(@path)
      differ(ideal, now, nocolor)
    end
def diff(nocolor = false)
      xml = Nokogiri::XML(File.open(@path), &:noblanks)
      ideal = xml.to_xml(indent: 2)
      now = File.read(@path)
      differ(ideal, now, nocolor)
    end
Ctor.
 +path+:: Path of it
 Return the difference, if any (empty string if everything is clean).