corsEnabled uses deprecated synchronous xhr

> Starting in Firefox 30, synchronous requests on the main thread have been deprecated due to their negative impact on performance and the user experience. Therefore, the async parameter may not be false except in a Worker.

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/open

From FileSaver.js:
```
function corsEnabled (url, opts) {
var xhr = new XMLHttpRequest()
// use sync to avoid popup blocker
xhr.open('HEAD', url, false)
try {
xhr.send()
} catch (e) {}
return xhr.status >= 200 && xhr.status <= 299
}
```