#444 fix is wrong

It is iconv that transpose arguments, not mb_convert_encoding.
_convertString encodes $string charset when Swift_Mime_MimePart charset ($this->getCharset()) is not utf-8. So mb_convert_encoding and iconv must convert from utf-8 to $charset.
bug #475 #444 fix is wrong (mimimi5)

This PR was merged into the 5.2-dev branch.

Discussion
----------

#444 fix is wrong

It is iconv that transpose arguments, not mb_convert_encoding.
_convertString encodes $string charset when Swift_Mime_MimePart charset ($this->getCharset()) is not utf-8. So mb_convert_encoding and iconv must convert from utf-8 to $charset.

Commits
-------

a96f3ee #444 fix is wrong