It seems like the Reminders app in iOS/macOS does this request as
the first thing when setting up an account, so it seems reasonable to
handle it for us.
This just returns the most basic current-user-principal now, but that
should hopefully be enough to continue the process.
I started using this project to export tasks over CalDav, more
specifically to Reminders on iOS/macOS. I quickly realized that
even if you specify that `SupportedComponentSet` contains `VTODO`, that
isn't reflected properly when doing the `PROPFIND`.
This patch should fix that, while keeping the behaviour of defaulting to
`VEVENT` for propfind. Also added some tests to make sure that I didn't
break anything (Which I hope I didn't 😅).
