[metal-router] using window.onbeforeunload does not work for popstate events

[bryceosterhaus]

I tried to reproduce with just Senna.js and was unable to reproduce, so I think it is specific to metal-router.

Reproduction Steps

1. Run demos
2. Navigate around to a few pages
3. Paste snippet in js console
   window.onbeforeunload = () => true
4. Click on the "back" arrow in the browser

Expected Result
Browser prompts a warning that asks before going back

Actual Result
Does not navigate, JS error metal.js:9173 Uncaught TypeError: event.preventDefault is not a function

[diegonvs]

I'd reproduced this behavior in senna examples and i think that is a senna issue. I'm working on this issue.