Main fetch should not make all about-scheme things same-origin

[bzbarsky]

The intent is to make it happen for about:blank (though see #384), and in the spec it's all the same since basic fetch will error out on other about: URLs. But in browsers, other about: URLs are implemented, and it would be safer from a security standpoint to only do the weird "make it same-origin" thing for about:blank, not all about: URLs.