Field alias not available in proxy store

Given the following mutation, I expect to have a `userId` field available in the proxy store.
```
const mutation = graphql`
mutation SetUserName($name: String!) {
setUserName(name: $name) {
user {
userId: id
name
}
}
}
`;
```

However, the alias isn't applied:
```
store.getRootField('setUserName').getLinkedRecord('user').getValue('userId') // undefined
```

Not sure if bug or planned for performance reasons. If it isn't a bug, I'll PR the docs, since it was a little bit of a gotcha.