title: @observable on numeric type doesn't create observable
body: <p>When trying to use @observable on a numeric type it doesn't create an Observable. This causes the problem where the computed value gets recalculated every time it is fetched.  I don't want the computed value to get recalculated unless the inputs have changed, what is the problem here?</p>
  
  <p><a href="https://jsbin.com/mofirujixu/1/edit?html,js,console" rel="nofollow noreferrer">https://jsbin.com/mofirujixu/1/edit?html,js,console</a></p>
  
  <pre><code>class Square {
    @observable width = 2;
  
    @computed get area() {
      console.log('calc area');
      return this.width * this.width;
    }
  }
  
  const square = new Square();
  //prints out 'calc area' each time
  console.log(square.area);
  console.log(square.area);
  console.log(square.area);
  </code></pre>