Preserving Return-Path header

When a mail bounces (e.g. mailbox full, receiver unknown, etc.) it is sometimes desirable to undertake further actions.

Use cases include purchase confirmation mails, user activation mail (think: Rails + Devise) et cetera. In these cases you'd define a `"#{user.encrypted_and_obsuscated_id}@bounces.example.com"` catch-all mailbox processing incoming the bounce mails.

Without this patch, the Return-Path header is attached to the Mail::Part, not the envelope causing the MTA on the receiving end to ignore this field completely (i.e. no bounce mail is created).