fix(decompress): join a repeated content-encoding header

[luantaraschi]

This relates to...

No open issue. Found while reading lib/interceptor/decompress.js.

Rationale

DecompressHandler.onResponseStart reads the header as a string:

const contentEncoding = headers['content-encoding']
...
const decompressors = this.#createDecompressionChain(contentEncoding.toLowerCase())

parseHeaders turns a repeated field into an array (lib/core/util.js, the val = [val] branch), and the JSDoc on this very method already types headers as Record<string, string | string[] | undefined>. An array is truthy, so #shouldSkipDecompression lets it through and .toLowerCase() throws:

TypeError: contentEncoding.toLowerCase is not a function

Reproduced against a raw socket server that writes the field twice:

HTTP/1.1 200 OK
Content-Type: text/plain
Content-Encoding: gzip
Content-Encoding: gzip

RFC 9110 section 5.3 lets a recipient combine repeated field lines into one by joining the values with commas, and that comma-joined form is exactly what #createDecompressionChain already splits and handles. So the fix is to join before anything else looks at the value.

The existing chain limit still applies afterwards: six repeated Content-Encoding lines join into six parts and hit the same maxContentEncodings guard as the single-line form, which the suite already covers.

Changes

onResponseStart joins an array-valued content-encoding with commas before the skip check and the chain build.

Test added to test/interceptors/decompress.js: a response with two Content-Encoding: gzip lines and a doubly gzipped body decompresses to the original text.

Features

N/A

Bug Fixes

The decompress interceptor no longer throws a TypeError when a response repeats Content-Encoding across field lines, and decompresses it the same way it decompresses the comma-joined single-line form.

Breaking Changes and Deprecations

None. Responses with a single Content-Encoding line take exactly the same path as before.

Status

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