Properly encode email headers fields with email addresses

RFC 2231 requires us to encode non-ascii characters but we must not encode
the email part of email addresses, only the real name part. Do this
properly now.

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>
Modify the test suite to use some non-ascii real names

This ensures that the code that encodes the real names
works properly, both for a single address and multiple addresses.

Signed-off-by: Raphaël Hertzog <hertzog@debian.org>
Ensure test emails are generated in a clean environment

$HOME/.gitconfig or /etc/gitconfig could lead to differing behaviour
of some commands used by git-multimail. In my case, it was "git log"
that gained an implicit --decorate.