[3.13] gh-78319: Fix implementation of IMAP APPEND UTF8 (GH-9436) (GH-139407)

gh-78319: Fix implementation of IMAP APPEND UTF8 (GH-9436)

Make UTF8 support for the IMAP APPEND command RFC 6855 compliant.
(cherry picked from commit 408154d64a7b3d43b05f44ea5c4cdf781bdd1352)

Co-authored-by: Gordon Messmer <gordon.messmer@gmail.com>