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

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.

Start at client.go line 341 and trace PutCalendarObject to see how its request is built. Add a way to send If-Match and If-None-Match, including If-None-Match: *, then verify that calendar objects can be created or updated safely under the requested conditions.

Written by the indexing model from the issue text.