Treat Accept-Encoding header as case-insensitive for gzip file check

[steverep]

What do these changes do?

Fixes issue in in server file response which does not convert Accept-Encoding header to lowercase before comparison.

Are there changes in behavior for the user?

No

Is it a substantial burden for the maintainers to support this?

No

Related issue number

Fixes 1st task of #8104

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_or_pr_num>.<type>.rst (e.g. 588.bugfix.rst)

  • if you don't have an issue number, change it to the pull request
    number after creating the PR

    • .bugfix: A bug fix for something the maintainers deemed an
      improper undesired behavior that got corrected to match
      pre-agreed expectations.
    • .feature: A new behavior, public APIs. That sort of stuff.
    • .deprecation: A declaration of future API removals and breaking
      changes in behavior.
    • .breaking: When something public is removed in a breaking way.
      Could be deprecated in an earlier release.
    • .doc: Notable updates to the documentation structure or build
      process.
    • .packaging: Notes for downstreams about unobvious side effects
      and tooling. Changes in the test invocation considerations and
      runtime assumptions.
    • .contrib: Stuff that affects the contributor experience. e.g.
      Running tests, building the docs, setting up the development
      environment.
    • .misc: Changes that are hard to assign to any of the above
      categories.

  • Make sure to use full sentences with correct case and punctuation,
    for example:

    Fixed issue with non-ascii contents in doctest text files
    -- by :user:`contributor-gh-handle`.

    Use the past tense or the present tense a non-imperative mood,
    referring to what's changed compared to the last released version
    of this project.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8b33fe6) 97.41% compared to head (16ba1e1) 97.41%.
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8127   +/-   ##
=======================================
  Coverage   97.41%   97.41%           
=======================================
  Files         107      107           
  Lines       32732    32732           
  Branches     3823     3823           
=======================================
  Hits        31885    31885           
  Misses        640      640           
  Partials      207      207           

Flag	Coverage Δ
CI-GHA	97.32% <100.00%> (ø)
OS-Linux	96.99% <100.00%> (ø)
OS-Windows	95.50% <100.00%> (ø)
OS-macOS	96.82% <100.00%> (ø)
Py-3.10.11	95.43% <100.00%> (ø)
Py-3.10.13	96.80% <100.00%> (-0.01%)	⬇️
Py-3.11.7	96.49% <100.00%> (ø)
Py-3.12.1	96.62% <100.00%> (ø)
Py-3.8.10	95.40% <100.00%> (ø)
Py-3.8.18	96.74% <100.00%> (ø)
Py-3.9.13	95.40% <100.00%> (ø)
Py-3.9.18	96.78% <100.00%> (ø)
Py-pypy7.3.15	96.33% <100.00%> (-0.01%)	⬇️
VM-macos	96.82% <100.00%> (ø)
VM-ubuntu	96.99% <100.00%> (ø)
VM-windows	95.50% <100.00%> (ø)

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[bdraco]

Thanks @steverep

[patchback]

Backport to 3.9: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.9/b97d9a97b239e33bc3f8df321defa853864a639e/pr-8127

Backported as #8130

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

[patchback]

Backport to 3.10: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.10/b97d9a97b239e33bc3f8df321defa853864a639e/pr-8127

Backported as #8131

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