fix blind readline of multipart epilogue

[terencehonles]

• fixes concern mentioned in PR fix #1526: multipart messages should consume their epilogue #1532

What do these changes do?

fixes concern mentioned in PR #1532

Are there changes in behavior for the user?

No

Related issue number

No

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• 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 #1581 into master will increase coverage by 0.02%.

@@            Coverage Diff             @@
##           master    #1581      +/-   ##
==========================================
+ Coverage   98.49%   98.51%   +0.02%     
==========================================
  Files          31       31              
  Lines        7219     7223       +4     
  Branches     1196     1197       +1     
==========================================
+ Hits         7110     7116       +6     
+ Misses         62       61       -1     
+ Partials       47       46       -1

Impacted Files	Coverage Δ
aiohttp/multipart.py	93.89% <100%> (+0.37%)	✅

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[terencehonles]

@kxepal This should probably ease your concerns 😉 , right?

[kxepal]

Indeed. Thanks, @terencehonles !

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