[select] Default value not working

#### Bug, feature request, or proposal:

> my-comp.html

```
<md-select placeholder="State" [(ngModel)]="myState">
   <md-option *ngFor="let state of states" [value]="state.code">{{ state.name }}</md-option>
</md-select>
```
> my-comp.ts

```
class MyComp {
  myState = 'AL';
  states = [{code: 'AL', name: 'Alabama'}];
}
```

#### What is the current behavior?
No item selected.