Use Kohana_UTF8 from version 3.3 to apply all the patches

Fixes Kohana_UTF8Test::test_clean with data set no.3
and Kohana_DebugTest::test_dump with data set no.6
Set and reset mb_substitute_character

To disable substituting illigal characters
with the default `'?'` character.

I think this solution is better than to set
`mb_substitute_character('none');` at the end of the file.

I also think this solution makes the setting of
`mb_substitute_character('none');` in `bootstrap.php` redundant.
replaced iconv with mb_convert_encoding in comments
Merge pull request #444 from enov/bug/4813/Kohana_UTF8

Fixes #4813 by Using Kohana_UTF8 from version 3.3 to apply all the patches