feat($http): add notify callback for loading requests

[chrisirhc]

Allows progress notifications for $http requests.

This makes use of the readyState 3 of XMLHttpRequest and uses the promises' notifications API which contains partial data except for IE 9 and below.

It's intentional that this doesn't call $apply since this may occur many times in a short span of time.
I'll add tests if this proves to be useful to add.

[caitp]

it's bitrotten :x

[caitp]

otherwise the fix doesn't look too bad, the only issue is that this doesn't deal with XHRUpload progress events --- in either case it's really hard to test this without E2E tests :(

[chrisirhc]

Sorry about the bad merge. Fixed.

Indeed, it'll be tricky to test in an actual environment even with E2E (uploads and downloads). The only way I thought of so far was to test it using readyState on the MockXhr object in the httpBackendSpec.

I didn't realize there were progress events on the XMLHttpRequest, but it looks like they don't actually contain partial data. But yes, it makes sense to make this work with upload progress events as well. I guess it can fire on both download and upload progress events as well as readyState = 3. Just need to figure out what exactly to call the notify() with for each of those cases. Right now, it's just calling it with the xhr object, while progress events actually have some useful information (loaded/total).

[pkozlowski-opensource]

@chrisirhc / All interested in this PR: based on the discussion in #1934 (staring from here) it seems like the general trend is to remove notifications (at least in the current form) from promises. As such we don't want to solve the problem of progress notifications using promise APIs.

I'm going to close this PR (and similar ones: #3606, #5874, #9258) and focus on the callback-based solution instead (similar to the one proposed in #7995).

We still do appreciate your effort on this issue which seems to be important to many people - so if you care about progress notifications being available in AngularJS please let's have discussion in #7995.