fix: accept mixed-case Content-Encoding values

[lprnmns]

Problem

HTTP content-coding names are case-insensitive, but request decompression currently compares the Content-Encoding value exactly. A client sending a valid mixed-case value such as Content-Encoding: GzIp receives a 415 response instead of having its gzip payload decompressed.

Fix

Normalize the request Content-Encoding value to lowercase before checking the configured supported encodings. The change is limited to the request decompression hook and adds regression coverage for mixed-case gzip.

Tests

• npm run test:unit -- test/global-decompress.test.js — passed (18 tests)
• npm run lint — passed
• npm test — passed (235 unit tests and 10 TypeScript assertions)
• npm run benchmark --if-present — passed; benchmark control stayed within the 10% tolerance
• git diff --check — passed

Compatibility

Lower-case request headers and forced request encodings keep their existing behavior. Only string Content-Encoding header matching is normalized; no public API or dependency changes are made.

Related issue

No current issue or pull request was found for mixed-case request Content-Encoding handling. This is an independent reproduction of the protocol case-sensitivity defect; it is distinct from open PR #425, which addresses response Accept-Encoding negotiation for issue #305.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin and the Code of conduct

[Copilot]

Pull request overview

Normalizes request Content-Encoding values for case-insensitive decompression matching.

Changes:

• Lowercases string encoding headers before lookup.
• Adds mixed-case gzip regression coverage.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
index.js	Normalizes request encoding values.
test/global-decompress.test.js	Tests mixed-case gzip handling.

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[lprnmns]

Good catch. Fixed in bb726a3: matching still uses the normalized value, while the callback and default error receive the original header value. The existing callback test now covers a mixed-case unsupported token; npm test and lint pass.