title: React not rerendering after mobx observer change
body: <p>Upon page load, I see "hi2"
  When I click the button, nothing happens. I tried with <code>setUser</code> as well.</p>
  
  <p>I suspect I'm just editing the props themselves and somehow the observable is not being triggered?</p>
  
  <p>See sample code of it not working here in a brand new rails/react environment: <a href="https://github.com/bufordtaylor/mobxtest" rel="noreferrer">https://github.com/bufordtaylor/mobxtest</a></p>
  
  <ol>
  <li>clone</li>
  <li>bundle</li>
  <li>rails s</li>
  <li>(in another process) ./bin/webpack-dev-server --host 127.0.0.1</li>
  <li>navigate to localhost:3000</li>
  </ol>
  
  <p>======================</p>
  
  <p>UPDATE:</p>
  
  <p>I've reduced it to it's basic form, eliminating possible import errors, Provider errors, or constructor errors.</p>
  
  <p>Here it is</p>
  
  <pre><code>import React from 'react'
  import ReactDOM from 'react-dom'
  import { observable, action, computed } from 'mobx';
  import { Provider, inject, observer } from 'mobx-react';
  
  
  class UserStore {
  
    @action setUser(val) {
      console.log(val);
      this.user = val;
    }
  
    @observable user = "default";
  }
  
  const userStore = new UserStore();
  
  @observer
  class Hello extends React.Component {
    render() {
      return (
        &lt;div&gt;
          hi2 {this.props.userStore.user}
          &lt;button onClick={this.props.userStore.setUser.bind(this,"fwefwe")}&gt;faew&lt;/button&gt;
        &lt;/div&gt;
      )
    }
  }
  
  document.addEventListener('DOMContentLoaded', () =&gt; {
    ReactDOM.render(
      &lt;Hello userStore={userStore} /&gt;,
      document.getElementById('app'),
    )
  })
  </code></pre>