• #833: Calendars throw exceptions when the sharing plugin is not enabled.
• #834: Return vCards exactly as they were stored if we don't need to convert
  in between versions.
• The zip release ships with sabre/vobject 4.1.0,
  sabre/http 4.2.1, sabre/event 3.0.0,
  sabre/uri 1.1.0 and sabre/xml 1.4.2.