树形控件开始拖拽事件没有回调参数产生，就是开始拖拽事件没有效果

[TTcom]

• I have searched the issues of this repository and believe that this is not a duplicate.

Version

1.3.4

Environment

window10家庭版、Google Chrome版本 70.0.3538.110（正式版本）、vue 2.5.22

Reproduction link

https://github.com/TTcom/tree-demo

Steps to reproduce

打开控制台，然后拖拽树形节点，控制台无法打印出开始拖拽事件的回调参数；

What is expected?

期望开始拖拽事件dragStart可以返回对应的回调参数；

What is actually happening?

当开始拖拽时没有回调参数产生

[tangjinzhou]

文档错误，dragxxx属于原生事件，使用全小写

[github-actions]

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