title: GraphQL returns null on mutation
body: <p>I'm using GraphQL to connect to multiple RESTful endpoints. I'm writing a mutation to update user details, however GraphiQL is showing the following response of null rather than the updated user ID and name...</p>
  
  <pre><code>{
    "data": {
      "updateUser": null
    }
  }
  </code></pre>
  
  <pre><code>  updateUser: (parent, args) =&gt; {
      const { id, name } = args;
      return fetch(`${url}/users/${id}`, {
        method: 'PUT',
        body: JSON.stringify({ id, name }),
        headers: { 'Content-Type': 'application/json' },
      })
        .then(res =&gt; res.json())
        .then(json =&gt; console.log(json));
    },
  </code></pre>
  
  <p>Thanks in advance.</p>