Associations which have not yet been fetched are undefined

See https://github.com/paularmstrong/normalizr/issues/267

If a normalized record looks like this...
```js
{
id: 'contact_123',
buckets: ['bucket_1', 'bucket_2', 'bucket_3']
}
```

...And `bucket_2` has not yet been fetched, the denormalized result for this contact will be:

```js
{
id: 'contact_123',
buckets: [
{ id: 'bucket_1', /* ...the rest... */ },
undefined, /* <- no bueno */
{ id: 'bucket_3', /* ...the rest... */ }
]
}
```

This isn't a problem in our app yet, since we always return nested resources. When we start optimizing performance and implementing sparse fields support, this will need to be handled.

Start by reading the linked Normalizr issue and tracing the normalization and denormalization path in this repository. No files or tests are named in the issue; completion criteria are not fully specified beyond handling an association that has not been fetched without producing an undefined entry.

Written by the indexing model from the issue text.