When users spell "cc:" in lowercase in the fake "header" in the
trailer part, "git send-email" failed to pick up the addresses from
there. As e-mail headers field names are case insensitive, this
script should follow suit and treat "cc:" and "Cc:" the same way.
* nz/send-email-headers-are-case-insensitive:
git-send-email: treat field names as case-insensitively