Fix dirtyKeys() and dirty(key:) on beforeSave when updating objects.

[nlutsenko]

Properly reconstruct the full object by applying values one-by-one to the original object if it's an object update.
There is another small piece missing - removing support for _Session triggers, which will unblock us on using the same approach for re-constructing dirtyKeys() on object creation in beforeSave().

Fixes #581, fixes #419

[drew-gross]

How is this going to interact with cloud code in other tests? If every cloud code test is modifying some global that seems like it could get messy fast.

[drew-gross]

Up to you if you want to do anything about that test comment.

[natanrolnik]

This should close #581 and #419

[nlutsenko]

@natanrolnik Yup, know about #581, but missed #419. Thanks, attaching the other one to the PR, so it's automatically closed when this is merged.

[nlutsenko]

Updated a little bit - the core logic didn't change, but unified all tests into a single describe() to be able to add before/after test to make sure we delete everything we setup.
Also, for a strange reason - had to enable strict mode so ES6 gets picked up. ¯_(ツ)_/¯

[facebook-github-bot]

@nlutsenko updated the pull request.

[drew-gross]

I'm not 100% sure, but I believe what is actually happening with use strict is that babel isn't applying to the tests, so use strict is enabling whatever fraction of es6 is supported natively in node, and that happens to be enough for this file.

[nlutsenko]

Huh, that might be true actually... Weird.

[btate]

Any idea when this will be in a tagged release so our package.json will pick it up?

[drew-gross]

Probably today or tomorrow.