Update the test for setting document.title to the empty string.

This was changed in https://www.w3.org/Bugs/Public/show_bug.cgi?id=24224 http://html5.org/tools/web-apps-tracker?from=8403&to=8404

Gecko/Trident match the new specification, WebKit/Blink match the old specification.
Merge pull request #523 from Ms2ger/empty-document-title

Update the test for setting document.title to the empty string; r=foolip