MimePart: fixed not escaped header before encoded into utf-8

[Daaarkling]

• bug fix
• BC break? no

Headers containing special characters such as Czech characters also need to be escaped before converting to utf-8.

Example header From: Kdo uteče, obědvá <[email protected]>

When google processes the email, it misparses the comma and because of that the DKIM signature verification fails and the email is rejected.