'Total' goes to 0 after responseStrategy subtract

Maybe need to verify this further, but the behavior I was noticing was something like this...

```js
collection.performAction('update', {
itemId: user.id,
data: {status: 'active'},
responseStrategy: 'subtract'
})
```

Result:
`collection.total => 0`
`collection.items.size => 20`

Start by reproducing the collection.performAction('update', ...) example with responseStrategy set to 'subtract', comparing collection.total with collection.items.size. Inspect the implementation of performAction and the subtract strategy to determine why total becomes 0, then add or update coverage so the total remains consistent with the collection after the response.

Written by the indexing model from the issue text.