fix(v-model): mutate original array for v-model multi checkbox (#2663)

fix(v-model): mutate original array for v-model multi checkbox (#2663)

Note: this change will break non-deep `watch` on the `v-model` bound array since the array is no longer replaced. This can be considered part of the Array watch changes in v3 as detailed at https://v3.vuejs.org/guide/migration/watch.html

This is unfortunate but unavoidable since the issue that it fixes is more important: `v-model` should definitely work with a non-ref reactive array.

fix #2662