R1:dc0d3f3fef23

Fix encodings for mime headers and body if not UTF-8

Summary:
If mails are not sent in UTF-8 we cannot just it verbatim, so we have to
encode it into UTF-8 if it is not the case. Mime headers use different
encodings like "quoted-printable", which we have to handle.

It looks like "Subject: =?iso-8859-1?Q?opr=E6t_s=E5_den_task?=", and can
be decoded by ##iconv_mime_decode##.

Furthermore the body of the email might be in various encodings as well,
which we attempt to pull from the content-type header of…