t/t9001-send-email.sh: sed - remove the i flag for s

The 'i' flag for the 's' command of sed is not specified by POSIX so
it is not portable.  Replace its usage by different and portable
syntax.

Signed-off-by: Marcel Telka <marcel@telka.sk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9001-send-email.sh

@@ -2526,7 +2526,7 @@ test_expect_success $PREREQ 'test forbidSendmailVariables behavior override' '
 
 test_expect_success $PREREQ '--compose handles lowercase headers' '
  write_script fake-editor <<-\EOF &&
- sed "s/^From:.*/from: edited-from@example.com/i" "$1" >"$1.tmp" &&
+ sed "s/^[Ff][Rr][Oo][Mm]:.*/from: edited-from@example.com/" "$1" >"$1.tmp" &&
  mv "$1.tmp" "$1"
  EOF
  clean_fake_sendmail &&