ICloud calendar support

[EasyHard]

icloud calendar CalDav has three places that are implemented differently.

1. It can return empty string "" on status:Dav for a calendar.
2. for each .ics, it returns two props, one without resource type.
3. It doesn't use Dav namespace for props

[jackkamm]

Thanks for investigating this.

I'd like to understand a bit better what is going on before merging this. Could you post a few examples of the HTTP responses you're getting from iCloud, that this patch fixes? In case it helps, I've added an extra level of debug verbosity, so that URL requests and responses are printed to *org-caldav-debug* when syncing. To use this, pull/merge the latest master branch, and set org-caldav-debug-level to 3.

Additional comments:

I think it sounds reasonable to treat empty string as 404 status, but should add a comment that this behavior is a workaround for iCalendar.

It seems strange to be adding in resourcetype tag into the org-caldav-url-dav-get-properties result, even when property isn't resourcetype (e.g when it's getetag instead).

Injecting "DAV:" as default namespace may be reasonable, but I'm not sure the propstat tag is the best place. Also it seems rather surprising that in the XML response from iCloud, the duplicate 404 entry you delete has xmlns="DAV:", but the 200 entry you keep is missing it.