AssertionError doesn't output values

When an assertion fails, nodeunit doesn't output the two values being compared, it just says 'AssertionError'

Test:

```
381:    test.equal('', undefined);
```

Output:

```
 AssertionError
    at Object.equal (/usr/local/lib/node_modules/nodeunit/lib/types.js:83:39)
    at Object.exports.update_normal_orderWithPurchaseOrdersReturns (/.../my-tests.js:381:7)
```
