`$pull` calls .update hook twice, once with null, once as it should be

Hello,

First, thanks for a great plugin!

Now to the problem: When I run a `$pull` command in mongodb, a strange thing happens. It calls my .update collection hook twice. The first time, with a `null` in place of the object, and the second time with the `null` removed.

First time

```
//this.previous
{
_id:"dsflaklfjslj",
myArr:[{_id:aeeoeio",val:"23"}]
}
//doc
{
_id:"dsflaklfjslj",
myArr:[null]
}
```

Now second time it is called

```
//this.previous
{
_id:"dsflaklfjslj",
myArr:[null]
}
//doc
{
_id:"dsflaklfjslj",
myArr:[]
}
```

This looks like a bug - it should only be called once with the null removed.

Thanks!

The issue names MongoDB's $pull operation and the collection update hook but no source file or test. Start by reproducing a $pull that removes an array element and trace the hook calls and intermediate documents. Done means the update hook runs once, with the removed element absent and no null intermediate state.

Written by the indexing model from the issue text.