Always upload using item's upload() method.

This allows to re-define item's upload function and perform some actions before item upload (as described [here](https://github.com/nervgh/angular-file-upload/issues/22).
Can be usefull while uploading to S3 because we need to perform some initial query to get upload url and upload parameters.
Merge pull request #1 from dmitry-dedukhin/patch-2

Always upload using item's upload() method.