[HTML] Get rid of the Haml::HTML::Node trickery.
[HTML] Get rid of the Haml::HTML::Node trickery.

Now that Hpricot only has one base class to support, we don't need to
turn backflips to allow a mixin to be included by multiple
superclasses.

nazar-pc/haml,haml/haml,stanhu/haml,haml/haml,tangposmarvin/haml,dg1dd1ngs/haml,concord-consortium/haml,keithpitty/haml,stanhu/haml,haml/haml,sideci-sample/sideci-sample-haml,macressler/haml,macressler/haml,tangposmarvin/haml,nazar-pc/haml,haml/haml,keithpitty/haml,dg1dd1ngs/haml,uniba/hamldoc_ja,amatsuda/haml