fixed the bug when try to download an array of files

```
it('should download an array of files', function (cb) {
        var src = [
            'https://www.google.se/images/srpr/logo4w.png',
            'https://ssl.gstatic.com/gb/images/k1_a31af7ac.png'
        ];
        var dest = 'tmp';
        var dl = download(src, dest);

        dl.once('close', function () {
            fs.statSync(dest + '/logo4w.png');
            fs.statSync(dest + '/k1_a31af7ac.png');
            cb();
        });
    });
}
```

I found that if I just run this test case, it will occur a error.
Actually the test case has some problems:
* close event emitted when just one file downloaded
* we have download the logo4w.png before:
```
    it('should download a file', function (cb) {
        var src = 'https://www.google.se/images/srpr/logo4w.png';
        var dest = 'tmp';
        var dl = download(src, dest);

        dl.once('close', function () {
            fs.stat(dest + '/logo4w.png', cb);
        });
    });
```

So that it will just download the k1_a31af7ac.png and pass the test case.