change argument order for isEqual?

[bgentry]

The config for isEqual currently takes a function with these args: (key, a, b). This threw me off because I assumed I could just use Ember.isEqual for this, but that only takes (a, b).

One solution which might be an easy fix would be to change the argument order to (a, b, key), which would make it work out-of-the-box with Ember.isEqual.

This would of course be a breaking API change but I think it'd be an improvement in the API.