Fix websocket disconnection errors

[tisztamo]

Close #722

[codecov-commenter]

Codecov Report

Merging #723 (83b152a) into master (209f10e) will increase coverage by 0.57%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
+ Coverage   76.29%   76.86%   +0.57%     
==========================================
  Files          37       37              
  Lines        2438     2442       +4     
==========================================
+ Hits         1860     1877      +17     
+ Misses        578      565      -13     

Impacted Files	Coverage Δ
src/WebSockets.jl	84.82% <75.00%> (-0.49%)	⬇️
src/Handlers.jl	62.96% <0.00%> (+0.46%)	⬆️
src/ConnectionPool.jl	80.74% <0.00%> (+0.68%)	⬆️
src/sniff.jl	84.96% <0.00%> (+0.75%)	⬆️
src/StreamRequest.jl	95.45% <0.00%> (+1.51%)	⬆️
src/Servers.jl	86.25% <0.00%> (+1.87%)	⬆️
src/access_log.jl	93.22% <0.00%> (+3.38%)	⬆️
src/IOExtras.jl	81.25% <0.00%> (+6.25%)	⬆️
src/parseutils.jl	73.33% <0.00%> (+20.00%)	⬆️

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[quinnj]

LGTM; thanks for the contribution!