[giow] (3) document.open() replaces when the current doc is the initial about:blank
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=17093
Affected topics: HTML Syntax and Parsing

git-svn-id: http://svn.whatwg.org/webapps@7738 340c8d12-0b0e-0410-8428-c7bf67bfef74