stringifyNumber(0) fails to stringify

I have to work my way with a legacy API that allows `0` as a valid `id` (don't ask). When wrapping this API with angular-data I bumped into:

http://errors.angularjs.org/1.2.25/ng/areq?p0=Expected%20key%20to%20be%20a%20string!%20Found%3A%20%7B0%7D.&p1=number

After applying this patch things started working.