Storing a dot-notated key for a firestore document should create a nested object

With firestore, when you set the following for an object:

```
firestore.doc(`foo`).set({
    'bar.thing': 3
})
```

You expect to get a document in Firestore with the following shape:
```
{
   bar: {
      thing: 3
   }
}
```

But currently the driver only stores the original dotted-string path to the original value. (`'bar.thing': 3`)