Fix issue #663 (FP)

Hello Igor,

as requested by mail, I created a pull-request to solve the problem described on the google groups (https://groups.google.com/forum/#!topic/select2/rWWybQ9qyks) and the bug tracker (#663 : https://github.com/ivaynberg/select2/issues/663).

My correction works no matter the level of the item.
I tested very well and I can assure you that it works perfectly !

Thank you for merge my version with the select2's master branch.
This is my first pull-request.

Thank's a lot.
Fabien