Fix incorrect warning in StreamReader.

[roganov]

What do these changes do?

I was getting a random warning when using request.multipart() concerning EOF state of StreamReader, though I was making exactly one read() for every BodyPartReader. The issue was that unread_data wasn't resetting _eof_counter.

Are there changes in behavior for the user?

No

Checklist

• [+ ] I think the code is well written
• [+] Unit tests for the changes exist
• [+] Documentation reflects the changes
• [+] If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• [+] Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • 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-io]

Codecov Report

Merging #2251 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2251      +/-   ##
=========================================
+ Coverage    97.2%   97.2%   +<.01%     
=========================================
  Files          39      39              
  Lines        7945    7946       +1     
  Branches     1378    1378              
=========================================
+ Hits         7723    7724       +1     
  Misses         98      98              
  Partials      124     124

Impacted Files	Coverage Δ
aiohttp/streams.py	99.07% <100%> (ø)	⬆️

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[asvetlov]

Thanks!

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