fixed a regression with the isCurrentAnimation function

somehow the `name` property of the current animation is being replaced
by `0` ! this needs a proper fix, but to do that, generally how data is
being stored in me.Sprite needs some rethinking/cleaning

src/renderable/sprite.js

@@ -375,6 +375,8 @@
                 this.current = this.anim[name];
                 this.resetAnim = resetAnim || null;
                 this.setAnimationFrame(this.current.idx);
+                // XXX this should not be overwritten 
+                this.current.name = name;
                 if (!_preserve_dt) {
                     this.dt = 0;
                 }