TpGroupMixin: allow no-op AddMembers for members, even without CAN_ADD
TpGroupMixin: allow no-op AddMembers for members, even without CAN_ADD

The spec says "Attempting to add contacts who are already members is
allowed; connection managers must silently accept this, without error."

Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib,Distrotech/telepathy-glib