Replace x.hasOwnProperty(y) with x[y] !== undefined

This seems to improve performance on the spinning balls test by around 5% on Chrome. jsperf says this is faster on both Firefox and Chrome desktop: http://jsperf.com/hasownproperty-vs-in-vs-undefined2

I think that this is the direction we want to go in the future because we may want to actually look on the prototype for keys, depending on how we want to implement immutable objects