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

*/
```

Reproduce the issue with the provided nodeunit test case, then trace the assertion-error formatting used by test.equal when actual is undefined. Done means the output includes the compared values, such as “undefined == 2”, while retaining the supplied assertion message.

Written by the indexing model from the issue text.