change `dirtyAttributes.pushObject` to `dirtyAttributes.addObject`

[scottmessinger]

This prevents the same attributes from appearing multiple times in the dirtyAttributes array

I'm not sure if this needs a test -- let me know and I'll add one.

[dwickern]

I noticed this too. I don't think there is any functional impact since removeObject will remove all instances of the key but it did confuse me.

[ebryn]

It would be nice to have a test that would fail prior to this change to make sure we don't regress.

[scottmessinger]

Thanks for the response! I'll go ahead and add a test.

[ebryn]

@scottmessinger ping

[scottmessinger]

Thanks for the ping. I haven't gotten a chance to do it, but I'm planning to.