use sendBeacon when available to support before/unload event sending

[schuyler1d]

https://github.com/segment-integrations/analytics.js-integration-segmentio.git
Depends on this library to send the json requests to track page/event tracking.

However, during beforeunload or unload events, many browsers (esp. Safari) do not send the tracking request. However the sendBeacon call reliably does and integration-segmentio does not seem to send anything more sophisticated than what's possible with sendBeacon.

This PR is conservative and only uses it when headers are nothing beyond a standard cors xhr request. If any additional headers are sent, then it fallsback to a regular xhr request.

[atrivedi1]

@f2prateek - If you have a sec, do you mind taking a look at this PR? in principle, it seems reasonable.

@schuyler1d is trying to track form-abandonment (e.g. via a tab/window close and/or manual location change). If he were to implement this manually, the code would be something like:

window.onbeforeunload = function(evt) {
window.analytics.track({event: 'form_dropoff', properties: {result: 'dropoff', ....}})
}

however, he's hoping Segment can handle this out of the box going forward. hence the PR.

[atrivedi1]

@schuyler1d - apologies for the delayed response!

after speaking to the engineering team, i can confirm that this is indeed something that is on our radar. in fact we even implemented a fix a while back: segment-integrations/analytics.js-integration-segmentio#19. however we had to revert this once we realized that beacon and localstorage-retry don't play nice with one another. rolling out a more robust version of localstorage-retry is something we are going to be actively working on in the next quarter or so and so our goal is to address this then.

i know that's probably not the answer you wanted to hear, but hopefully that at least provides you with a bit more context!

[schuyler1d]

@atrivedi1 Thanks for the context. If there's an issue I can track to see when this lands, please let me know. Otherwise, I wish you and your team the best -- looking forward to an even better segment!

[schuyler1d]

I'd be interested to know if there's movement on this?