RuleDB/Notify: properly en-/decode the mail subject

RuleDB/Notify: properly en-/decode the mail subject

we need to mime decode the subject after reading it, so that we get
the 'real' subject instead of the (possibly) mime encoded one (which
might be base64 or quoted-printable encoded). To get a proper subject in
the notification mail again, we have to encode it again before passing
it MIME::Entity->build

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-By: Stoiko Ivanov <s.ivanov@proxmox.com>
Reviewed-By: Stoiko Ivanov <s.ivanov@proxmox.com>