If an Unknown message is being constructed for sending, the content
may be empty, since the msgtype and body are exposed as C++ properties.
Previously, the to_json function assumed that the content could be
parsed as valid json, when in fact an empty content would throw an error.

Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

* Add confetti support

- Added version check
- Added 'invitee' to m.call.invite
- Added 'party_id' to all VoIP events
- Added new events
  - m.call.select_answer
  - m.call.reject
  - m.call.negotiate
- Added reason codes to m.call.hangup
- Added custom struct for session description objects