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 :)