mbox: do not copy Content-Type into exported mbox

mbox: do not copy Content-Type into exported mbox

Daniel reports a patch + comment combination that breaks in
git am. The patch reports a Content-Type of US-ASCII, while
the comment adds a Ack with UTF-8 characters. The exported
mbox contains both the original Content-Type, and a UTF-8
Content-Type that we set. However, because the US-ASCII one
occurs later, git am honours it instead of ours, and chokes
on the UTF-8 characters.

Strip out any subsequent Content-Type:s. We normalise things
to UTF-8 and should not allow it to be overridden.

Add a test for this, based on the original report.

Reported-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Axtens <dja@axtens.net>