scroll组件内部放textarea 内容过多时，无法滑动

### Version
1.9.3

### Reproduction link
[https://l44llqp9j9.codesandbox.io/](https://l44llqp9j9.codesandbox.io/)

### Steps to reproduce

### What is expected?
textarea 内容多时可以滚动

### What is actually happening?
textarea不可以滚动

### Dependencies Version
vue: 2.5.17-beta.0
better-scroll: 1.11.1

---
BScroll.prototype._move = function (e) {
if (!this.enabled || this.destroyed || eventType[e.type] !== this.initiated) {
return;
}

if (this.options.preventDefault) {
e.preventDefault();
}

缺少对preventDefaultException的判断，以前放个div，over-flow=auto也无法滚动