bug(module:button): class ant-btn-icon-only is not applied to the icon-only-button

[LouisSung]

Reproduction link

https://stackblitz.com/edit/ng-zorro-antd-start-uaapgo

Steps to reproduce

• I've tried to use the stackblitz to reproduce this bug, but seems like it's not yet supported [email protected] (with Angular 9.1.1 & Ivy)

• However, this bug is easy to reproduce as the sample code

  • <button nz-button nzType="danger"><i nz-icon nzType="caret-up"></i></button>
    <button nz-button nzType="primary" class="ant-btn-icon-only"><i nz-icon nzType="caret-up"></i></button>
    <!-- ^^^ should be added by ng-zorro automatically (works fine @8.5.2) ^^^ -->

  • 

What is expected?

Buttons containing icon only should apply class ant-btn-icon-only as the blue button

What is actually happening?

Buttons containing icon only do NOT apply class ant-btn-icon-only as the red button

Environment	Info
ng-zorro-antd	9.0.0-beta.4
Browser	chrome:80

[vthinkxie]

duplicated #4955