Button in a primary colored toolbar does not get colored white

```
<ion-header>
    <ion-toolbar color="primary">
       <ion-button class="closeButton" clear>
           close
        </ion-button>
    </ion-toolbar>
</ion-header> 
```
In `ionic-angular` a button inside a `primary` toolbar will have `color: white` on it. This does not currently happen in `core`