wocky_roster_add_contact_async: replace current details if needed
wocky_roster_add_contact_async: replace current details if needed

When we have an add contact and the contact is already on the roster,
we should check the details are the same, and if they are not, update
the contact, _replacing_ the current details (name, group) with
those from the add call.

freedesktop-unofficial-mirror/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,freedesktop-unofficial-mirror/wocky,noonien-d/wocky,noonien-d/wocky