title: Completed property in Angular Material Stepper doesn't seem to work as expected
body: <p>Here is a <a href="https://stackblitz.com/edit/mat-horizontal-stepper-3i5s1j?file=app%2Fapp.component.html" rel="nofollow noreferrer">stackblitz</a> of this behavior.  What I expect to see is that when the Complete button is clicked, the first step is complete, so the icon would change to "10".  However, there is no change, and even when moving to the second step, the icon is "create".  Am I misunderstanding how the <code>completed</code> property is supposed to be working, or is there a bug in the Material code?</p>