dbus-account-plugin: update internal cache correctly
dbus-account-plugin: update internal cache correctly

The backend-makes-changes test replaces an typed parameter with an
untyped parameter. We correctly signalled the change to MC,
but didn't update the internal cache; the rather strange way in
which MC receives attributes/parameters masked this bug.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=27727

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control