Correctly handle newline detection not only for open boundary but for close mark too

[asvetlov]

#3520 introduced an error on the master.
It adds a test where closing multipart boundary is present before opening one.
The test has a conflict with newline style (\n vs \r\n) detection.
This PR fixes the conflict.

@kxepal please review.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@0ad5d90). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #3758   +/-   ##
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  Coverage          ?   97.93%           
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  Files             ?       43           
  Lines             ?     8569           
  Branches          ?     1375           
=========================================
  Hits              ?     8392           
  Misses            ?       74           
  Partials          ?      103

Impacted Files	Coverage Δ
aiohttp/multipart.py	97.03% <100%> (ø)

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[arthurdarcet]

Thanks, sorry I missed this!

[kxepal]

👍

[kxepal]

It adds a test where closing multipart boundary is present before opening one.

But is this a well formed multipart? I fear with allowing empty multiparts we just also allowed handling malformed multiparts.

[asvetlov]

At least the test has a first multipart chunk like --boundary--\r\n while --boundary\r\n was expected.
IIRC in conversation @arthurdarcet convinced that at lease urllib3 and email.message allows a lone closing boundary