Treat an empty Transfer-Encoding list as no transfer codings

[ericmj]

A Transfer-Encoding field with no list elements, such as an empty value or ",", failed the response with :empty_token_list and closed the connection. For a response to HEAD this happened before the rule that such a response has no body, and a comma-only field between two nonempty Transfer-Encoding fields failed the response instead of being ignored.

RFC 9110 5.6.1 allows a #transfer-coding list with zero elements, so such a field now adds no transfer coding when the framing is chosen. A response with only an empty Transfer-Encoding and no Content-Length has no framing, so its body is read until the connection closes (RFC 9112 6.3). The field still counts as present: with Content-Length it's an error, and it makes an HTTP/1.0 response, including a 1xx one, close the connection (RFC 9112 6.1).

Transfer-Encoding values are parsed only where they decide the framing, combined into one value as on the request side, so a malformed value no longer fails a response to HEAD, a 204 or 304 response, or a 2xx response to CONNECT, whose Transfer-Encoding RFC 9110 9.3.6 says to ignore. Where it does fail, the error comes after the :headers event (with :stream_headers the field itself can be emitted first) and not while the header section is incomplete, other header errors in the same section are reported instead of it, and with Content-Length the error is :transfer_encoding_and_content_length instead of {:invalid_token_list, value}.

On the request side, a Transfer-Encoding field with no codings passed to request/5 with a streamed body counts as no codings: when chunked is appended to such a last field it replaces the empty value.

[github-actions]

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Coverage: 90.514%

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• Patch coverage: 9 of 9 lines across 2 files are fully covered (100%).

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Coverage Stats

Relevant Lines:	2003
Covered Lines:	1813
Line Coverage:	90.51%
Coverage Strength:	712.93 hits per line

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