Fix Rmail editing with reapplying encoding to message body

Fix Rmail editing with reapplying encoding to message body

* lisp/mail/rmailedit.el (rmail-cease-edit):  If no
content-type in edited headers, look for one in original
headers and add it to edited headers.  (Bug #26918)
Use a marker to track start of new body, so that
content-transfer-encoding gets applied only to body.  (Bug #27353).
Ensure blank line at end of message after encoding, not
before.