ContactCapabilities: standardize Gabble's behaviour of explicitly fixing the handle type
ContactCapabilities: standardize Gabble's behaviour of explicitly fixing the handle type

This matches telepathy-qt4's client-side implementation, makes
implementation in clients significantly easier, and allows us to define
semantics for other target handle types in future.

freedesktop-unofficial-mirror/telepathy__telepathy-spec,TelepathyIM/telepathy-spec