title: Mobx-react does not update component when observable.map value is changed
body: <p>In my store ..</p>
  
  <pre><code>@observable dots = observable.map({
    '0x001': extendObservable({ x: 2000, y: 2000 }),
    '0x002': extendObservable({ x: 5000, y: 5000 }),
    '0x003': extendObservable({ x: 10000, y: 10000 }),
  });
  
  @action updateDot(id, properties) {
    const dot = this.dots.get(id);
    this.dots.set(id, extendObservable(Object.assign(dot, properties)));
  }
  </code></pre>
  
  <p>When I update the dot values  ..</p>
  
  <pre><code>store.updateDot('0x002', {
    x: 200,
    y: 300,
  });
  </code></pre>
  
  <p>My components do not react ..</p>
  
  <pre><code>@inject('store') @observer
  export class IndexComponent extends Component {
  
    render() {
      const { store } = this.props;
      const { dots } = store;
  
      return &lt;Map dots={dots} /&gt;
    }
  
  }
  </code></pre>
  
  <p>And Map ..</p>
  
  <pre><code>@observer
  class Map extends Component {
    render() {
      const { dots } = this.props;
      // NOT CALLED WHEN UPDATE DOTS
      return &lt;Matrix dots={dots} /&gt;
    }
  }
  </code></pre>
  
  <p>And Matrix ...</p>
  
  <pre><code>@observer
  class Matrix extends Component {
    render() {
      const { dots, ...rest } = this.props;
      // NOT CALLED WHEN UPDATE DOTS
      return &lt;div ...rest &gt;&lt;/div&gt;
    }
  }
  </code></pre>