Simplify StreamWriter ctor

[asvetlov]

No description provided.

[codecov-io]

Codecov Report

Merging #2958 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2958      +/-   ##
=========================================
+ Coverage      98%     98%   +<.01%     
=========================================
  Files          40      40              
  Lines        7583    7584       +1     
  Branches     1324    1324              
=========================================
+ Hits         7432    7433       +1     
  Misses         48      48              
  Partials      103     103

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	97.35% <ø> (ø)	⬆️
aiohttp/web_fileresponse.py	98.01% <ø> (ø)	⬆️
aiohttp/http_writer.py	100% <100%> (ø)	⬆️
aiohttp/web_protocol.py	91.86% <100%> (ø)	⬆️
aiohttp/test_utils.py	98.86% <100%> (ø)	⬆️

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[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new