Bring back fast enumeration to JSONModelArray

Bring back fast enumeration to JSONModelArray, lost in commit c4ffee63b64a71c1319b6b5149f60ee88c5f92b8. Based on a [sample from Apple](https://developer.apple.com/library/mac/samplecode/FastEnumerationSample/Introduction/Intro.html#//apple_ref/doc/uid/DTS40009411-Intro-DontLinkElementID_2).