Fixed header Mime encoding failing using multi-byte characters

When using multi-byte characters (i.e. Kanji/Katakana in japanese), `iconv_mime_encode()` function fails converting to *quoted printable*, generating a notice and returning `false`. This prevents using the Mail component properly in Japanese or other multi-byte based languages.

* [PHP related issue](https://bugs.php.net/bug.php?id=53891) (probably related to underlying iconv library).
* [eZ Publish related issue](https://jira.ez.no/browse/EZP-21567)

Added a regression test.