web: make HTTP field names case-insensitive

I'm opening this PR to (hopefully!) stimulate a discussion. In brief,
I'd like to amend the gRPC-Web protocol docs to encourage
implementations to follow HTTP semantics and compare HTTP field names
case-insensitively.

The gRPC-Web specification is a nicely-designed way for proxies to
expose standard HTTP/2 gRPC servers to clients using less
tightly-controlled HTTP stacks, such as web browsers. To serve that
goal, it seems valuable to have the gRPC-Web specification follow [RFC
9110 (HTTP
Semantics)](https://www.rfc-editor.org/rfc/rfc9110.html#name-field-names).
Like previous RFCs, 9110 specifies that "field names are
case-insensitive." However, the current gRPC-Web specification requires
that servers and proxies "use lower-case header/trailer names" on the
wire. In principle, mandating casing on the wire is normal for HTTP/2
and fine (if unusual) for HTTP/1.1; however, it encourages
implementations to violate HTTP semantics and require lower-case names
when _reading_ headers and trailers.

I'd like to loosen the gRPC-Web specification to permit any casing on
the wire for HTTP/1.1. I'd also like to emphasize that gRPC-Web
implementations ought to follow standard HTTP semantics when _reading_
fields and compare names case-insensitively. This amendment does not
affect the correctness of Envoy (which may continue to use lower-case
field names) or the `grpc-web` Javascript project (which already treats
field names case-insensitively, thanks to the Web Platform's `Headers`
interface). However, it does clarify that `grpc-dart` is _incorrect_ in
relying on lower-case field names.

Relates to improbable-eng/grpc-web#228,
https://github.com/bufbuild/connect-go/issues/453/, and
grpc/grpc-dart#594.

doc/PROTOCOL-WEB.md

@@ -48,7 +48,12 @@ Content-Type
 HTTP wire protocols
 
 1. support any HTTP/*, with no dependency on HTTP/2 specific framing
-2. use lower-case header/trailer names
+2. send lower-case header/trailer names over HTTP/2 (following [RFC
+   9113](https://www.rfc-editor.org/rfc/rfc9113#section-8.2-2)), send
+   header/trailer names using any case over HTTP/1.1 (following [RFC
+   9110](https://www.rfc-editor.org/rfc/rfc9112.html#section-5-1)), and compare
+   header/trailer names case-insensitively (following [RFC
+   9110](https://www.rfc-editor.org/rfc/rfc9110.html#section-5.1-3))
 3. use EOF (end of body) to close the stream
 
 ---