Fix outlook charset problem

[bakicdj]

When I send email from outlook desktop client v10, with utf8 chars (šđčćž), the library converts them to question marks. This is because outlook sends message in iso-8859-2 charset, and mb_convert_encoding expect from charset to be the real one, but this if block converts original charset to utf-8. This is how I fix it.

[tomsommer]

This is not the correct fix. See #148