fix: check writer is closing in AIOKafkaConnection.send

[dimastbk]

Changes

Fixes #1038

Checklist

• [ ] I think the code is well written
• [x] 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[bot]]

Codecov Report

Attention: Patch coverage is 94.23077% with 3 lines in your changes missing coverage. Please review.

Project coverage is 95.10%. Comparing base (29b58db) to head (132604b).

Files with missing lines	Patch %	Lines
tests/test_conn.py	93.87%	1 Missing and 2 partials :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1044   +/-   ##
=======================================
  Coverage   95.09%   95.10%           
=======================================
  Files         114      114           
  Lines       16981    17028   +47     
  Branches     1579     1583    +4     
=======================================
+ Hits        16148    16194   +46     
  Misses        485      485           
- Partials      348      349    +1     

Flag	Coverage Δ
cext	94.91% <94.23%> (+<0.01%)	:arrow_up:
integration	94.83% <94.23%> (-0.01%)	:arrow_down:
purepy	94.91% <94.23%> (+<0.01%)	:arrow_up:
unit	53.54% <82.69%> (+0.44%)	:arrow_up:

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[dimastbk]

@ods Hi! Could you take a look at this PR?