Fixing Return-Path header in outgoing mail

This http://dev.rubyonrails.org/ticket/7572 patch allows setting the
Return-Path header for outgoing messages. When set, this will be used
as envelope from instead of the From: address.

This in turn allows for automated bounce handling and the VERP
technique, very useful for automatically picking up failed
registration confirmations.

Looking for three +1s, or comments. Or both.

http://dev.rubyonrails.org/ticket/7572