Expose xhr object on promise

[olalonde]

See issue #89

[dgraham]

The XMLHttpRequest object is an implementation detail of this fetch implementation that cannot be exposed to callers.

[olalonde]

@dgraham I understand that but is there any way to get upload progress event out of fetch? Can't seem to find any other than this hack.

[dgraham]

Fetch doesn't support progress events, but XMLHttpRequest does.

var xhr = new XMLHttpRequest()
xhr.open('POST', '/uploads')
xhr.onload = function() {
  console.log(xhr.status, xhr.responseText)
}
xhr.onerror = function() {}
xhr.upload.onprogress = function (event) {
  if (event.lengthComputable) {
    var percent = Math.round((event.loaded / event.total) * 100)
    console.log(percent)
  }
}