send-email: use UTF-8 rather than utf-8 for consistency

send-email: use UTF-8 rather than utf-8 for consistency

The rest of the git source has been converted to use upper-case character
encoding names to assist older platforms.  The charset attribute of MIME
is defined to be case-insensitive, but older platforms may still have an
easier time dealing with upper-case rather than lower-case.  So do so for
send-email too.

Update t9001 to handle the changes.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>