Fix #1 "Handling base64 attached forwarded emails"

Handle email forwarded as attachment with "base64" encoding or any
other supported encoding by
javax.mail.internet.MimeUtility.decode(java.io.InputStream,
java.lang.String) to avoid NullPointerException errors.

Indeed based on the RFC 2046, forwarded emails (Content-Type:
message/rfc822) are supposed to be encoded as "7bit", "8bit", or
"binary" but email clients such as Microsoft Outlook
don't respect that RFC.