title: can't do POST to api, error 400 using fetch
body: <pre><code>fetch('/auth/signup', {
        method: 'POST',
        body: this.state.user,
        headers: {
          'Content-Type': 'application/json'
        },
      })
      .then(function(response) {
        return response.json();
      })
      .then(data =&gt; {
        console.log(data);
      })
  </code></pre>
  
  <p>this.state.user is basically an object like <code>{user: {name:"someone"}</code> and my server side code simply do this</p>
  
  <pre><code>router.post('/signup', (req, res, next) =&gt; {
    console.log(req.body) // empty object
  }
  </code></pre>
  
  <p>I think something is wrong with <code>fetch</code>, didn't see any data been pass in the network tab.</p>