bringing up to date with json2.js - compat change for IE7

this change reflects current json2.js code.

How I discovered this:
I bundled the json2ify npm module using requirejs for some client-side code.
Using a JS error tracking service I observed a consistent bug, occurring only in IE7: "JSON" is undefined.
I looked at the up-to-date version of crockford's code, and his implementation changed from what was included in this library to how I've modified it.
Please update the npm library asap.

Ben