caldav: add support for If-Match and If-None-Match in PutCalendarObject

[jech]

From client.go line 341:

	// TODO: add support for If-None-Match and If-Match

Without this, it is impossible to update a calendar in a safe manner. At the very least, we need to be able to specify If-None-Match: * in order to safely create an event.

[Opisek]

I am currently unable to update events, due to getting an error saying 409 Conflict: <error xmlns="DAV:"><no-uid-conflict xmlns="urn:ietf:params:xml:ns:caldav"></no-uid-conflict></error>, when attempting to run PutCalendarObject on a modified copy of an event that's already in the calendar.

Is this related?

Edit: That was my mistake. I was accidentally changing the UID of the event when trying to update. In that case, could you please still elaborate, though, what you mean by "safe manner"? If it feels dire enough to me, I might look into contributing this feature in.