Fix #1985: Keep a reference to ClientSession in response object

[asvetlov]

Fix for #1985

@fafhrd91 is the change good for you?

[codecov-io]

Codecov Report

Merging #1988 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1988      +/-   ##
==========================================
+ Coverage   97.05%   97.05%   +<.01%     
==========================================
  Files          38       38              
  Lines        7672     7674       +2     
  Branches     1339     1339              
==========================================
+ Hits         7446     7448       +2     
  Misses        102      102              
  Partials      124      124

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	97.19% <100%> (+0.02%)	⬆️
tests/autobahn/client.py	97.07% <0%> (-0.02%)	⬇️

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[fafhrd91]

you should set session to None in _cleanup_writer, just in case

[asvetlov]

Done.
Honestly I think it's not necessary: request and response have an unidirection reference to session but the session doesn't keep references to req/rep. Thus we have no circle and regular decref should release everything very well.

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