fix(module:pulltorefresh): fix when up pull, then down pull, emit ref…

[nuonuoge]

…resh error

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-mobile/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
[x] No

Other information

[codecov]

Codecov Report

Merging #248 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage    86.8%   86.77%   -0.03%     
==========================================
  Files         229      229              
  Lines        7344     7344              
  Branches     1037     1037              
==========================================
- Hits         6375     6373       -2     
- Misses        565      566       +1     
- Partials      404      405       +1

Impacted Files	Coverage Δ
...nents/pull-to-refresh/pull-to-refresh.component.ts	72.12% <0%> (-1.22%)	⬇️

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[Guoyuanqiang]

close #244