fix(module:icon): fix icon classname writeback

[wzhudev]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #2259 into master will decrease coverage by <.01%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #2259      +/-   ##
==========================================
- Coverage   95.78%   95.78%   -0.01%     
==========================================
  Files         485      485              
  Lines       12146    12142       -4     
  Branches     1629     1628       -1     
==========================================
- Hits        11634    11630       -4     
  Misses        152      152              
  Partials      360      360

Impacted Files	Coverage Δ
components/icon/nz-icon.directive.ts	81.33% <80%> (-0.95%)	⬇️

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