[CalDAV] Perform free-busy-query on the correct principal
Currently, when a participant is added to a meeting, an RFC6638 outbox-based freebusy query is performed.
If that fails,
ecb_caldav_get_free_busy_from_principal_sync is invoked, which searches for the principal matching the attendee's email address, and performs a REPORT-based free-busy-query on that principal...
Except it doesn't! The
href found in the search is ignored, and eventually freed without being used. Instead, NULL is passed as the URI to
e_webdav_session_report_sync, causing it to default to querying (and in turn displaying) the user's own free/busy information.
This patch queries the
href found via the search.