Add status 103 and 308 and fix status messages

This is going to sound crazy, and I should have made a note of that in a comment, but there are actually WebDAV clients that break when the casing of `200` is not `OK`. Don't ask me how I found out :P It was a horrible process :P

Also, the `Checkpoint` needs at least some kind of official source. I've never heard of it ;)