net/http: only support "chunked" in inbound Transfer-Encoding headers

This is a security hardening measure against HTTP request smuggling.
Thank you to ZeddYu for reporting this issue.

We weren't parsing things correctly anyway, allowing "identity" to be
combined with "chunked", and ignoring any Transfer-Encoding header past
the first. This is a delicate security surface that already broke
before, just be strict and don't add complexity to support cases not
observed in the wild (nginx removed "identity" support [1] and multiple
TE header support [2]) and removed by RFC 7230 (see page 81).

It'd probably be good to also drop support for anything other than
"chunked" in outbound TE headers, as "identity" is not a thing anymore,
and we are probably off-spec for anything other than "chunked", but it
should not be a security concern, so leaving it for now. See #38867.

[1]: https://hg.nginx.org/nginx/rev/fe5976aae0e3
[2]: https://hg.nginx.org/nginx/rev/aca005d232ff

Change-Id: If17d0827f9c6167a0b19a158e2bc5844ec803288
Reviewed-on: https://go-review.googlesource.com/c/go/+/231418


Reviewed-by: Katie Hockman <katie@golang.org>