title: `data` is undefined in apollo useMutation Promise
body: <p>Why is <code>data</code> undefined in Promise even though it appears in html below?</p>
  
  <p>I dont want to use <code>.then((data) =&gt;</code></p>
  
  <pre><code>const LoginPage: React.SFC&lt;{}&gt; = () =&gt; {
      const [login, { loading, data, error }] = useMutation&lt;Q, V&gt;(MUTATION)
  
      const onSubmit = event =&gt; {
          event.preventDefault()
          login({
              variables: {
                  email,
                  password
              }
          }).then(() =&gt; {
              console.log(data)         // &lt;-- undefined
          }).catch(() =&gt; {
              console.log(error)        // &lt;-- OK
          })
      }
  
      return &lt;div&gt;
          &lt;form onSubmit={onSubmit}&gt; ... &lt;/form&gt;
  
          {data &amp;&amp; &lt;pre&gt;{JSON.stringify(data)}&lt;/pre&gt;}          //  &lt;-- OK
      &lt;/div&gt;
  }
  </code></pre>