bug: invoke the callback whether the connection is open or not

[panjf2000]

Fixes #465

[codecov]

Codecov Report

Patch coverage: 32.25% and project coverage change: -0.24 ⚠️

Comparison is base (59268af) 77.06% compared to head (00907d7) 76.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #466      +/-   ##
==========================================
- Coverage   77.06%   76.82%   -0.24%     
==========================================
  Files          23       23              
  Lines        2411     2421      +10     
==========================================
+ Hits         1858     1860       +2     
- Misses        420      428       +8     
  Partials      133      133              

Flag	Coverage Δ
unittests	76.82% <32.25%> (-0.24%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
connection_windows.go	64.52% <0.00%> (ø)
connection_unix.go	71.20% <45.45%> (-1.68%)	⬇️

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