Correct File.clone() treatment of File.stats

[hughsk]

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 :)

[hughsk]

@Contra using the freshly published clone-stats module now, look good to merge?

[yocontra]

@hughsk the module needs tests and so does this PR

[hughsk]

@Contra done, for both.