Slight change to the event replies so that they work with Outlook

Outlook has decided that it will not act on an event reply unless the ics file is in a multiplart/mixed part. If not, the person who scheduled the meeting will get the reply mail with a note saying that the person accepted or rejected the event, but the Outlook calendar won't be updated.

This fix changes this so that the reply is more friendly to Outlook's shenanigans.