JSON.stringify spacer needs to be third parameter when using json2.js

This fixes an error when Breeze's exportMetadata calls JSON.stringify when json2.js is used.

JSON.stringify syntax according to Mozilla is:

    JSON.stringify(value[, replacer [, space]])

If you try to pass in `space` without also passing in `replacer`, I guess it works in most implementations, but json2.js for one will throw an error.

See also
http://stackoverflow.com/a/17342752/31563