Fix change event for select with multiple
Fix change event for select with multiple

The `change` and `input` events didn't trigger because the old and new
value in the `val` watcher always ended up being identical. Fixing this
by cloning `val` before modifying it.

wffranco/vue-strap,BiodataMiningGroup/vue-strap,lotestudio/vue-strap,lotestudio/vue-strap,wffranco/vue-strap,BiodataMiningGroup/vue-strap