fix missing cleanup, if response socket is no longer writeable (issue 1547)

[Azlond]

Relates to issue #1547, see my comment there

[codecov]

Codecov Report

Merging #1593 (d7570c8) into master (7d8955b) will not change coverage.
The diff coverage is 100.00%.

❗ Current head d7570c8 differs from pull request most recent head bae657d. Consider uploading reports for the commit bae657d to get more accurate results

@@           Coverage Diff           @@
##           master    #1593   +/-   ##
=======================================
  Coverage   99.60%   99.60%           
=======================================
  Files           5        5           
  Lines         507      507           
  Branches      142      142           
=======================================
  Hits          505      505           
  Misses          2        2           

Impacted Files	Coverage Δ
lib/application.js	98.24% <100.00%> (ø)

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[miwnwski]

LGTM, though would you mind adding a test for this?

[Azlond]

I don't really know how to write tests for koa, so not sure if I can be of help with that.