http2: compare Connection header value case-insensitively

The case was ignored elsewhere in this file, but not in this place.

Fixes golang/go#23699

Change-Id: I222092c10aab33d652df5d028cf93716955c20a5
Reviewed-on: https://go-review.googlesource.com/122588
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/transport.go

@@ -747,7 +747,7 @@ func checkConnHeaders(req *http.Request) error {
 	if vv := req.Header["Transfer-Encoding"]; len(vv) > 0 && (len(vv) > 1 || vv[0] != "" && vv[0] != "chunked") {
 		return fmt.Errorf("http2: invalid Transfer-Encoding request header: %q", vv)
 	}
-	if vv := req.Header["Connection"]; len(vv) > 0 && (len(vv) > 1 || vv[0] != "" && vv[0] != "close" && vv[0] != "keep-alive") {
+	if vv := req.Header["Connection"]; len(vv) > 0 && (len(vv) > 1 || vv[0] != "" && !strings.EqualFold(vv[0], "close") && !strings.EqualFold(vv[0], "keep-alive")) {
 		return fmt.Errorf("http2: invalid Connection request header: %q", vv)
 	}
 	return nil

http2/transport_test.go

@@ -2022,6 +2022,11 @@ func TestTransportRejectsConnHeaders(t *testing.T) {
 			value: []string{"close"},
 			want:  "Accept-Encoding,User-Agent",
 		},
+		{
+			key:   "Connection",
+			value: []string{"CLoSe"},
+			want:  "Accept-Encoding,User-Agent",
+		},
 		{
 			key:   "Connection",
 			value: []string{"close", "something-else"},
@@ -2032,6 +2037,11 @@ func TestTransportRejectsConnHeaders(t *testing.T) {
 			value: []string{"keep-alive"},
 			want:  "Accept-Encoding,User-Agent",
 		},
+		{
+			key:   "Connection",
+			value: []string{"Keep-ALIVE"},
+			want:  "Accept-Encoding,User-Agent",
+		},
 		{
 			key:   "Proxy-Connection", // just deleted and ignored
 			value: []string{"keep-alive"},