Update UploadHandler.php

In the post() method, if the $upload variable is null ($_FILES[$this->options['param_name']] does not exist), the old code will still try to handle the upload as if $_FILES is a one-dimensional array.
Merge pull request #3378 from Aszze/patch-1

Update UploadHandler.php
Updated patch version for last merge.
Merge branch 'master' into gh-pages