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

*/
```