Update xhr.overrideMimeType() to the latest spec
Update xhr.overrideMimeType() to the latest spec

This allows us to pass the updated overridemimetype-invalid-mime-type.htm web platform test.

tmpvar/jsdom,snuggs/jsdom,mzgol/jsdom,snuggs/jsdom,mzgol/jsdom,tmpvar/jsdom,Zirro/jsdom,Zirro/jsdom,mzgol/jsdom