when `actual` is undefined, AssertionError == Assertion Message.

example:

```
module.exports.myTest = function (test) {
    test.expect(1);

    test.equal(undefined, 2, "some message")

    test.done();
}

/*

$ nodeunit test.js 

test.js
✖ myTest

Assertion Message: some message
AssertionError: some message // should say undefined == 2

*/
```
