Fixed ClientSession initialization warning

[AraHaan]

This fixes an issue with ClientSession throwing a warning when feeding
an event loop into it when calling it from a normal function. This
should help silence annoying warnings for libraries that "lazy"
initializes ClientSession and properly closes the session and handles
it. Now those libraries would have no need for useless bug reports.

What do these changes do?

Fixes the warning when calling a ClientSession instance with an event loop that is passed to the class.

Are there changes in behavior for the user?

None

Related issue number

#1468 (comment)

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[codecov-io]

Codecov Report

Merging #1586 into master will increase coverage by <.01%.

@@            Coverage Diff             @@
##           master    #1586      +/-   ##
==========================================
+ Coverage   98.54%   98.54%   +<.01%     
==========================================
  Files          32       32              
  Lines        7265     7276      +11     
  Branches     1208     1210       +2     
==========================================
+ Hits         7159     7170      +11     
  Misses         61       61              
  Partials       45       45

Impacted Files	Coverage Δ
aiohttp/client.py	100% <100%> (ø)	✅

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[fafhrd91]

add extra code just to satisfy tests? -1
proper solution is to create session inside task.

[Rapptz]

Not that I'm condoning this PR, but @fafhrd91 the original PR was proposed to be modified: #1468 (comment)

[fafhrd91]

i think this PR is too much for andrew's comment.
in any case i addressed andrew's comment in aae6f8b

[AraHaan]

Ok, but you mind also adding my name in the CONTRIBUTORS.txt file to from this PR? @fafhrd91
And also this to CHANGES.rst:

- Fix ClientSession is not aware of TestClient #1499

Which was forgotten when the fix to #1499 got pushed.

[fafhrd91]

done

[AraHaan]

thanks.

[lock]

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