Merge #136: Ensure valid encoding on attachments in forwarded mail

Fixes #135. Forwarded attachments/parts may contain invalid encoding
resulting in failure when forwarding email. This fix uses fix_encoding!
to ensure valid encoding on all parts.

Squashed commit of the following:

commit 83b057e77b9409dba0a50dab9baf94727d8e5e0f
Author: Gaute Hope <eg@gaute.vetsj.com>
Date:   Fri Aug 23 09:06:04 2013 +0200

    fix encoding of fields and attachments as well, specify output stream as UTF-8

commit b89bf34e2e47940a7853403a118a11070e0ee8c3
Author: Gaute Hope <eg@gaute.vetsj.com>
Date:   Fri Aug 23 08:40:54 2013 +0200

    Revert "fix encoding of all parts"

    This reverts commit 98f9a7c609828a9c57ab4db2fe9c2c826f6d201b.

commit 98f9a7c609828a9c57ab4db2fe9c2c826f6d201b
Author: Gaute Hope <eg@gaute.vetsj.com>
Date:   Fri Aug 23 08:38:45 2013 +0200

    fix encoding of all parts

commit 43e80873e5fd7705a6218071eaff30d53b25515a
Author: Gaute Hope <eg@gaute.vetsj.com>
Date:   Wed Aug 21 08:45:06 2013 +0200

    Fix encoding on message to be forwarded