results.failures is undefined if there are no failures

[Jameskmonger]

When running a group of tests with 4 assertions, all of which pass, the following listener gives an output I wouldn't expect:

input.on('complete', function (results) {
    console.log(results);
});

Results here is:

{ ok: true, count: 4, pass: 4, plan: { start: 1, end: 4 } }

Whereas if one of the tests failed, it would be:

{ ok: true, count: 4, failures: [ array of failures here ], pass: 3, plan: { start: 1, end: 4 } }

I would expect failures to be an empty array if there were none. This would make it easier to iterate over.

[isaacs]

Fixed by #30