http2: case insensitive handling for 100-continue

rfc 9110, section 10.1.1 states that the Expect field value is
case-insensitive.

Fixes golang/go#57824

Change-Id: Ie0e2662c58a2933087e0d35935c04ec61026a41d
Reviewed-on: https://go-review.googlesource.com/c/net/+/463096
Auto-Submit: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

http2/server.go

@@ -2192,7 +2192,7 @@ func (sc *serverConn) newWriterAndRequestNoBody(st *stream, rp requestParam) (*r
 		tlsState = sc.tlsState
 	}
 
-	needsContinue := rp.header.Get("Expect") == "100-continue"
+	needsContinue := httpguts.HeaderValuesContainsToken(rp.header["Expect"], "100-continue")
 	if needsContinue {
 		rp.header.Del("Expect")
 	}

http2/server_test.go

@@ -2332,7 +2332,7 @@ func TestServer_Response_Automatic100Continue(t *testing.T) {
 	}, func(st *serverTester) {
 		st.writeHeaders(HeadersFrameParam{
 			StreamID:      1, // clients send odd numbers
-			BlockFragment: st.encodeHeader(":method", "POST", "expect", "100-continue"),
+			BlockFragment: st.encodeHeader(":method", "POST", "expect", "100-Continue"),
 			EndStream:     false,
 			EndHeaders:    true,
 		})