Fix MB characters handling in split

The PR fixes using multibyte characters in the ```split```-filter

Since I messed up the rebase of https://github.com/fabpot/Twig/pull/1446 I created a new PR.
Let me know if this something that is kind of wanted, thanks!

Good point, most of the tests I added in the commit are already covered there. I moved the one case that was in here but not in the ```Fixtures/``` one