Properly resolve absolute URLs in compat

Towards compatibility with Serene Notes
Support request.responseType = 'arraybuffer'

Towards compatibility with Flappy Bird
Remember filenames of URLs created after document load
Mock out img.src

For compatibility with flappy-birds
Revert "XMLHttpRequest: prepare responses to .html requests"

This reverts commit 89b5915f53647919522d272467235a212e7526ad.

It broke Firetext.