This needs to be case-insensitive.

The RFC says this SHOULD be lower case, but interpreters SHOULD ignore
the case. This fixes auto handling of iTIP responses from MS clients such as
Outlook and WP.
Per the EAS specs, this is the client's job.

Fixes sending multiple iTIP replies to the same request.
Fix sending server changes in certain cases.

When there are incoming changes, server changes, and a missing
GETCHANGES tag, server changes were not sent.
Send proper status code when srcmsg is not found.