#12147: make send_message correctly handle Sender and Resent- headers.

Original patch by Nicolas Estibals.  My tweaks to the patch were mostly
style/cosmetic, and adding more tests.
diff --git a/Misc/ACKS b/Misc/ACKS
index 74409f7..45b4042 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -265,6 +265,7 @@
 Ben Escoto
 Andy Eskilsson
 Stefan Esser
+Nicolas Estibals
 Stephen D Evans
 Carey Evans
 Tim Everett