Support relative <img src=> URLs.

`img.src` is an absolute URL. jsdom's `img.src` getter produces this URL by resolving the `<img src=>` attribute value relative to the document's URL—which is bogus unless we explicitly pass a correct URL to `jsdom.env()` to start with.

(With this change, inliner works on <http://people.mozilla.org/~jorendorff/es6-draft.html>.)