Default JSON secure to true in JSON.decode to avoid eval()

- Make the `secure` parameter default to true, to avoid using eval() as
defaultbehaviour for JSON.decode
Commented at
http://mootools.net/docs/core/Utilities/JSON#comment-1157192420

- Added specs to cover hazarous behaviour if wanted
- Added 1.4compat layer comments to the adition
- Fiddle with changes and specs: http://jsfiddle.net/qYkc7/