bug #1521 Fix MB characters handling in split (1emming)

This PR was squashed before being merged into the 1.16-dev branch (closes #1521).

Discussion
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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!

Commits
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8cde52d Fix MB characters handling in split

diff --cc lib/Twig/Extension/Core.php
Simple merge