fix($httpBackend): complete the request on timeout

When using the [timeout attribute](https://xhr.spec.whatwg.org/#the-timeout-attribute) and an XHR
request times out, browsers trigger the `timeout` event (and execute the XHR's `ontimeout`
callback). Additionally, Safari 9 handles timed-out requests in the same way, even if no `timeout`
has been explicitly set on the XHR.
In the above cases, `$httpBackend` would fail to capture the XHR's completing (with an error), so
the corresponding `$http` promise would never get fulfilled.

Note that using `$http`'s `timeout` configuration option does **not** rely on the XHR's `timeout`
property (or its `ontimeout` callback).

Fixes #14969
Closes #14972