Add close_boundary option to write closing boundary on multipart writer

[james4388]

What do these changes do?

Add close_boundary option in write method of MultipartWriter to support Streaming content

Are there changes in behavior for the user?

Allow MJpeg (and other streaming that use multipart/x-mixed-replace)

Related issue number

#3104

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.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-io]

Codecov Report

Merging #3106 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3106      +/-   ##
==========================================
+ Coverage   98.05%   98.06%   +<.01%     
==========================================
  Files          43       43              
  Lines        7834     7837       +3     
  Branches     1353     1354       +1     
==========================================
+ Hits         7682     7685       +3     
  Misses         56       56              
  Partials       96       96

Impacted Files	Coverage Δ
aiohttp/multipart.py	96.02% <100%> (ø)	⬆️
aiohttp/web_response.py	98.19% <0%> (ø)	⬆️

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[asvetlov]

Looks good.

I've tweaked documentation a little, will merge when green.

[asvetlov]

thank you

[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