git-send-email: treat field names as case-insensitively

git-send-email: treat field names as case-insensitively

Field names like To:, Cc:, etc. are case-insensitive; use a
case-insensitive regexp to match them as such.

Previously, git-send-email would fail to pick-up the addresses when
in-body "fake" headers with different cases (e.g. lowercase "cc:")
are manually inserted to the messages it was asked to send, even
though the text will still show them.

Signed-off-by: Nickolai Zeldovich <redacted>
Signed-off-by: Junio C Hamano <redacted>