Add "unused" parameter to all setters

* ECMAScript ExpectedArgumentCount for a setter is always 1

Change motivated by failure of JS parser used by browserify:
Error: Parsing file /home/krose/.nave/installed/0.10.5/lib/node_modules/jsdom/lib/jsdom/level2/html.js: Line 503: Unexpected token )

I've since decided that browserify is hopeless, but it can't hurt to be standards-compliant as much as possible.