Fix null returns for play()

The play() method in the AnimationManager could return null for a valid animation name.

src/animation/AnimationManager.js

@@ -227,6 +227,7 @@ Phaser.AnimationManager.prototype = {
                     this.currentAnim.paused = false;
                     return this.currentAnim.play(frameRate, loop, killOnComplete);
                 }
+                return this.currentAnim;
             }
             else
             {