Reject promise on request timeout

[dgraham]

Firefox and Chrome trigger the request's onerror handler when a timeout occurs. Safari triggers the ontimeout handler.

This can be observed by making a request to an unroutable address:

var xhr = new XMLHttpRequest()
xhr.onload = console.log.bind(console, 'loaded')
xhr.onerror = console.log.bind(console, 'errored')
xhr.ontimeout = console.log.bind(console, 'timeout')
xhr.open('GET', 'http://10.255.255.1')
xhr.send()

Fixes #294.

@mislav @fabioberger

[fabioberger]

Thanks for putting this together dgraham! I hope it gets merged soon!

[staabm]

Would be great to have it documented in the source which lines covering which browser

[mislav]

👍 🚢