tweaks for handling parts in reply

[Jameson Graef Rollins]

This is a small set of tweaks to remove unneccessary "Non-text part:"
lines in reply, for parts that really don't need to be mentioned.

This patch set is not really related to the series that it is being
sent in reply to, but it has been worked on top of the message/rfc822
rework of the multipart test, so it's being sent in reply to that
message.

jamie.