Do not destroy the ripple when it is disabled

Author: willshowell

src/demo-app/button/button-demo.html

@@ -101,4 +101,8 @@
     <button md-button>Check<md-icon class="md-24">favorite</md-icon></button>
     <button md-button>Last<md-icon>navigate_before</md-icon></button>
   </section>
+  <section>
+    <button md-raised-button [disabled]="toggleDisable" (click)="toggleDisable = true">Disable</button>
+    <button md-button [disabled]="!toggleDisable" (click)="toggleDisable = false">Disable</button>
+  </section>
 </div>

src/demo-app/button/button-demo.ts

@@ -10,4 +10,5 @@ import {Component} from '@angular/core';
 export class ButtonDemo {
   isDisabled: boolean = false;
   clickCounter: number = 0;
+  toggleDisable: boolean = false;
 }

src/lib/button/button.html

@@ -1,6 +1,7 @@
 <span class="mat-button-wrapper"><ng-content></ng-content></span>
-<div md-ripple *ngIf="!_isRippleDisabled()" class="mat-button-ripple"
+<div md-ripple class="mat-button-ripple"
      [class.mat-button-ripple-round]="_isRoundButton || _isIconButton"
+     [mdRippleDisabled]="_isRippleDisabled()"
      [mdRippleCentered]="_isIconButton"
      [mdRippleTrigger]="_getHostElement()"></div>
 <!-- the touchstart handler prevents the overlay from capturing the initial tap on touch devices -->