Parse boundary from headers while making POST

[weastur]

What do these changes do?

Now, while making POST request with formdata and explicitly set header Content-Type the boundary will be used from header param if present. Without these changes, HTTP servers will fail to parse formdata because they will be waiting for boundary from Content-Type header, but aiohttp will generate random.

Are there changes in behavior for the user?

Only fixes behavior when boundary explicitly set. Another cases works as earlier.

Related issue number

#5558

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]

Codecov Report

Merging #5575 (9c213dc) into master (2dfc801) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5575   +/-   ##
=======================================
  Coverage   93.32%   93.33%           
=======================================
  Files         102      102           
  Lines       30140    30160   +20     
  Branches     2696     2700    +4     
=======================================
+ Hits        28129    28149   +20     
  Misses       1837     1837           
  Partials      174      174           

Flag	Coverage Δ
unit	93.26% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
aiohttp/client_reqrep.py	98.03% <100.00%> (+0.01%)	⬆️
aiohttp/formdata.py	98.93% <100.00%> (+0.01%)	⬆️
aiohttp/multipart.py	96.12% <100.00%> (+0.01%)	⬆️
tests/test_client_request.py	99.57% <100.00%> (+<0.01%)	⬆️
tests/test_formdata.py	100.00% <100.00%> (ø)
tests/test_multipart.py	100.00% <100.00%> (ø)

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[asvetlov]

LGTM, thanks!

[patchback]

Backport to 3.8: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply dccf502 on top of patchback/backports/3.8/dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39/pr-5575

Backporting merged PR #5575 into master

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/aio-libs/aiohttp.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.8/dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39/pr-5575 upstream/3.8

4. Now, cherry-pick PR Parse boundary from headers while making POST #5575 contents into that branch:

   $ git cherry-pick -x dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39

   If it'll yell at you with something like fatal: Commit dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39 is a merge but no -m option was given., add -m 1 as follows intead:

   $ git cherry-pick -m1 -x dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Parse boundary from headers while making POST #5575 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.8/dccf502fbcb4bd0f5ab1c7dd5ae1fe69541f4f39/pr-5575

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[aio-libs-github-bot]

💔 Backport was not successful

The PR was attempted backported to the following branches:

• ❌ 3.8: Commit could not be cherrypicked due to conflicts