Normalize Gmail API message header names to lowercase before lookup so
gmail get/search/reply populate to/subject/from regardless of the casing
the message was stored with. Emit conventional MIME header casing
(To/Subject/Cc/From) on send and reply.
Fixes#34806
Co-authored-by: Donovan Yohan <donovan-yohan@users.noreply.github.com>