Make some changes to javascript scheme url evaluation to bring it
closer to
https://html.spec.whatwg.org/multipage/browsing-the-web.html#navigating-across-documents

- Evaluate the js before the page load so that it happens in the
  correct `window` global.
- If the result is not a string, the response should be a 204.

This required saving some data in load_data, since it's not
possible to modify the response at the point where we're evaluating
the js.