fix(module:modal): fix multiple nzOnCancel calls

[dmytroyarmak]

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?

Double click on cancel button or on "mask" of a modal window causes multiple invocations of ngOnCancel.

Issue Number: #958

What is the new behavior?

Only first click on cancel button or "mask" invokes ngOnCancel.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #1590 into master will decrease coverage by <.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1590      +/-   ##
==========================================
- Coverage      96%   95.99%   -0.01%     
==========================================
  Files         466      466              
  Lines       10954    10956       +2     
  Branches     1448     1450       +2     
==========================================
+ Hits        10516    10517       +1     
  Misses        123      123              
- Partials      315      316       +1

Impacted Files	Coverage Δ
components/modal/nz-modal.component.ts	96.03% <75%> (-0.47%)	⬇️

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