JSON.stringify(undefined) produces an empty string. Patch to clarify output

I spent a good half hour trying to figure out what was wrong with my assertion before I figured out what the output meant.

Before patch:

```
AssertionError: [{"id":1,"name":"john doe"}] deepEqual
    at Object.deepEqual (/Users/chiper/ChiperSoft/JS/seaquell/node_modules/nodeunit/lib/types.js:83:39)
    at /Users/chiper/ChiperSoft/JS/seaquell/tests/seaquell.js:120:8
    ...
```

After patch:

```
AssertionError: [{"id":1,"name":"john doe"}] deepEqual undefined
    at Object.deepEqual (/Users/chiper/ChiperSoft/JS/seaquell/node_modules/nodeunit/lib/types.js:83:39)
    at /Users/chiper/ChiperSoft/JS/seaquell/tests/seaquell.js:120:8
    ...
```