fix: progress event expects byte size not time

[xg-wang]

Spec:

• https://xhr.spec.whatwg.org/#the-send()-method
• https://xhr.spec.whatwg.org/#concept-event-fire-progress

fix #274 , supersedes #275
cc @seanjohnson08 @mike-north

---

TODO:

• all existing tests continue to pass
• we write a new test that would have failed before this code change, and now passes due to the alteration of how the progress events are handled
• we include great documentation around "handling uploads with Pretender"

[seanjohnson08]

Looks great! wondering if you can fix the typo in "elapsed"? :)

[mike-north]

awesome. I'm fine with treating this as a "fix" and making an accompanying patch version bump, providing

• all existing tests continue to pass
• we write a new test that would have failed before this code change, and now passes due to the alteration of how the progress events are handled
• we include great documentation around this aspect of Pretender, so others don't have to work quite so hard to figure things out as @seanjohnson08 did

[xg-wang]

Looks great! wondering if you can fix the typo in "elapsed"? :)

Good catch! Fixed now.

[xg-wang]

@mike-north I pushed 2 commits, first failling test looks like:

PhantomJS 2.1.1 (Linux 0.0.0) pretender invoking async requests with `onprogress` upload events in the upload  trigger a progress event each 50ms FAILED
	ProgressEvent has total of requestBody byte size
	Expected: 9
	Actual: 300

Also added a section in readme.