title: Apollo GraphQL subscription response doesn't handle nested queries
body: <p>I have the following GraphQL subscription that <strong>works fine</strong>:</p>
  
  <pre><code>subscription voucherSent($estId: Int!) {
    voucherSent(estId: $estId) {
      id
      name
      usedAt
      sentAt
    }
  }
  </code></pre>
  
  <p>But the following sends an "Cannot read property 'User' of undefined" error</p>
  
  <pre><code>subscription voucherSent($estId: Int!) {
    voucherSent(estId: $estId) {
      id
      name
      usedAt
      sentAt
      owner {
        id
        username
      }
    }
  }
  </code></pre>
  
  <p>Does Apollo GraphQL subscription handle nested queries?</p>
  
  <p>Here is my resolver code:</p>
  
  <pre><code>return models.Voucher.update({
      sentAt: moment().format(),
      usedIn: args.sentTo,
    }, { where: { id: args.id } })
      .then(resp =&gt; (
        models.Voucher.findOne({ where: { id: args.id } })
          .then((voucher) =&gt; {
            pubsub.publish(VOUCHER_SENT, { voucherSent: voucher, estId: voucher.usedIn });
            return resp;
          })
      ))
  </code></pre>