fix: check writer is closing in AIOKafkaConnection.send

[dimastbk]

Changes

Fixes #1038

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.02%. Comparing base (01c60cd) to head (b6fec2d).

Files	Patch %	Lines
tests/test_conn.py	94.11%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1044   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         114      114           
  Lines       16971    17019   +48     
  Branches     2771     2782   +11     
=======================================
+ Hits        16126    16173   +47     
  Misses        497      497           
- Partials      348      349    +1     

Flag	Coverage Δ
cext	91.94% <94.44%> (+0.01%)	⬆️
integration	94.67% <94.44%> (-0.01%)	⬇️
purepy	94.49% <94.44%> (-0.01%)	⬇️
unit	53.29% <83.33%> (+0.41%)	⬆️

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