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)
```

Start in lib/types.js at the assertion code around line 83 and reproduce the shown test.equal('', undefined) case. Update the failure output so the compared values are shown, then verify that the example no longer reports only “AssertionError”.

Written by the indexing model from the issue text.