Correct File.clone() treatment of File.stats

Cloning the stats object would remove its prototype, meaning it would lose methods normally available on fs.Stats
instances, such as Stats.isDirectory() or Stats.isFile()

This was giving me errors when cloning vinyl instances and then piping them to `gulp.dest()` streams, which do a directory check if the `stats` property is present:

```
/Users/hughsk/Desktop/repos/gulp/lib/createOutputStream/index.js:19
      if (file.stat && file.stat.isDirectory()) {
                                 ^
TypeError: Object #<Object> has no method 'isDirectory'
    at /Users/hughsk/Desktop/repos/gulp/lib/createOutputStream/index.js:19:34
    at /Users/hughsk/Desktop/repos/gulp/node_modules/mkdirp/index.js:38:26
    at Object.oncomplete (fs.js:107:15)
```

This should fix the problem :)