title: Map modification not triggering store update
body: <p>I am adding NGRX to my application in angular 4. </p>
  
  <p>I have an issue with store updating over a Map Object.</p>
  
  <p>My store is a <code>Map&lt;string,MyObj&gt;;</code> </p>
  
  <p>When I update the map, the store is updated, but the observable for changes do not fire on my store.</p>
  
  <pre><code>const ACTION_Increment = (state: Map&lt;string,MyObj&gt; = initialState, action:Increment) =&gt; {
      if(!state.get(action.payload.id)) state.set(action.payload.id, action.payload.myobj);
      else state.set(action.payload.id, {...state.get(action.payload.id), ...action.payload.myobj});
  
  
      console.log("Status Updated : ", state)
      return state;
  }
  
  const ACTION_Decrement = (state: Map&lt;string,MyObj&gt; = initialState, action:Increment) =&gt; {
      console.log("Status Updated : ", state)
      return new Map&lt;string,MyObj&gt;();
  }
  </code></pre>
  
  <p>Here an exemple on <a href="https://stackblitz.com/edit/ngrx-seed-oq9ldt" rel="nofollow noreferrer">StackBlitz</a>.</p>
  
  <p>If you click + I add a random element to my store, but no trigger are fired, if I press - I reset the map so assign a new object, and in this case, the trigger is fired.</p>
  
  <p>How can I make it trigger in booth situation ? </p>