Fix KeepAlive timeout handling in default dispatcher

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 41.66667% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 90.66%. Comparing base (c9c85f2) to head (6dae563).
Report is 1 commits behind head on master.

Files	Patch %	Lines
ntex-io/src/dispatcher.rs	50.00%	4 Missing ⚠️
ntex-io/src/ioref.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #351      +/-   ##
==========================================
+ Coverage   90.62%   90.66%   +0.03%     
==========================================
  Files         197      197              
  Lines       31646    31689      +43     
==========================================
+ Hits        28680    28731      +51     
+ Misses       2966     2958       -8     

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