Add unicode support in cart product name.

I added the full unicode support for the product cart name if utf8 is enabled in the project.
Issues related: 
https://github.com/EllisLab/CodeIgniter/issues/78
https://github.com/EllisLab/CodeIgniter/issues/57
http://ellislab.com/forums/viewthread/186579

I did not added documentation in the manual because anyway the rule has never been documented.