textarea计算中文所占的字符数错误

[duqian42707]

Bug Report or Feature Request (mark with an x)

[x] Bug report -> please search issues before submitting
[ ] Feature request
[ ] Documentation issue or request

Current behavior

textarea的属性weui-cn设置为大于2的数字，一个中文仍按2个字符处理，请看示例。

Expected behavior

weui-cn设置为3时，一个中文按3个字符处理。

Environment

Angular version: X.Y.Z

ngx-weui version: X.Y.Z

Browser:
- [x] Chrome (desktop) version 68.0.3440.106（正式版本） （64 位）
- [x] Firefox version 61.0.2 (64 位)
- [x] Safari (desktop) version 11.0.3 (13604.5.6)
- [?] IE version XX

Others: