Address some review comments.

--HG--
extra : amend_source : 5cd00caf17bdae19e0193b4329fbd088ed8e843d
Add Return-Recipt-To as a well-known equivalent to Disposition-Notification-To.
Implement preferred naming when outputting structured headers.
Merge master.
Add date headers to structured headers (no tests yet, though!).
Fix up 2231-encoding and add some (but not enough) tests.