Add document.charset alias
Add document.charset alias

See https://github.com/whatwg/dom/commit/6941936bd06438f84ad91d131e2e89ab0f1f7a45. This makes the latest web platform tests pass.

Zirro/jsdom,mbostock/jsdom,nicolashenry/jsdom,fiftin/jsdom,tmpvar/jsdom,Joris-van-der-Wel/jsdom,fiftin/jsdom,mbostock/jsdom,snuggs/jsdom,Joris-van-der-Wel/jsdom,jeffcarp/jsdom,fiftin/jsdom,Joris-van-der-Wel/jsdom,mbostock/jsdom,Zirro/jsdom,snuggs/jsdom,Sebmaster/jsdom,tmpvar/jsdom,mzgol/jsdom,mzgol/jsdom,jeffcarp/jsdom,nicolashenry/jsdom,Sebmaster/jsdom,mzgol/jsdom,nicolashenry/jsdom,jeffcarp/jsdom,Sebmaster/jsdom