Fix headers being mutated if passed to web.Response as a CIMultiDict

[bdraco]

What do these changes do?

If CIMultiDict is passed in we need to make a copy to avoid mutating it. In some cases we used to copy these twice which was fixed in #10045 but for this case that was the only copy being made and the source of this regression.

fixes #10670

[codspeed-hq]

CodSpeed Performance Report

Merging #10672 will not alter performance

_{Comparing header_mutate_fix (397bfd8) with master (d637ec9)}

Summary

✅ 51 untouched benchmarks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.71%. Comparing base (83a9df5) to head (397bfd8).
Report is 3088 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10672      +/-   ##
==========================================
+ Coverage   98.10%   98.71%   +0.61%     
==========================================
  Files         126      125       -1     
  Lines       37601    37445     -156     
  Branches     2132     2075      -57     
==========================================
+ Hits        36887    36963      +76     
+ Misses        545      335     -210     
+ Partials      169      147      -22     

Flag	Coverage Δ
CI-GHA	98.59% <100.00%> (+0.60%)	⬆️
OS-Linux	98.25% <100.00%> (+0.57%)	⬆️
OS-Windows	96.19% <100.00%> (+1.43%)	⬆️
OS-macOS	97.36% <100.00%> (+0.54%)	⬆️
Py-3.10.11	97.27% <100.00%> (+0.56%)	⬆️
Py-3.10.16	97.81% <100.00%> (+0.54%)	⬆️
Py-3.11.11	97.90% <100.00%> (+0.52%)	⬆️
Py-3.11.9	97.36% <100.00%> (+0.55%)	⬆️
Py-3.12.9	98.36% <100.00%> (+0.58%)	⬆️
Py-3.13.2	98.34% <100.00%> (+0.58%)	⬆️
Py-3.9.13	97.14% <100.00%> (+0.54%)	⬆️
Py-3.9.21	97.68% <100.00%> (+0.54%)	⬆️
Py-pypy7.3.16	86.45% <100.00%> (+2.83%)	⬆️
VM-macos	97.36% <100.00%> (+0.54%)	⬆️
VM-ubuntu	98.25% <100.00%> (+0.57%)	⬆️
VM-windows	96.19% <100.00%> (+1.43%)	⬆️

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[patchback]

Backport to 3.11: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 35c979b on top of patchback/backports/3.11/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672

Backporting merged PR #10672 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/3.11/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672 upstream/3.11

4. Now, cherry-pick PR Fix headers being mutated if passed to web.Response as a CIMultiDict #10672 contents into that branch:

   $ git cherry-pick -x 35c979be79dcc75ecbf7270ccc9fde264e5e2948

   If it'll yell at you with something like fatal: Commit 35c979be79dcc75ecbf7270ccc9fde264e5e2948 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 35c979be79dcc75ecbf7270ccc9fde264e5e2948

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix headers being mutated if passed to web.Response as a CIMultiDict #10672 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.11/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672

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.

[patchback]

Backport to 3.12: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 35c979b on top of patchback/backports/3.12/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672

Backporting merged PR #10672 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/3.12/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672 upstream/3.12

4. Now, cherry-pick PR Fix headers being mutated if passed to web.Response as a CIMultiDict #10672 contents into that branch:

   $ git cherry-pick -x 35c979be79dcc75ecbf7270ccc9fde264e5e2948

   If it'll yell at you with something like fatal: Commit 35c979be79dcc75ecbf7270ccc9fde264e5e2948 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 35c979be79dcc75ecbf7270ccc9fde264e5e2948

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Fix headers being mutated if passed to web.Response as a CIMultiDict #10672 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.12/35c979be79dcc75ecbf7270ccc9fde264e5e2948/pr-10672

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.