Match Content-Type charset case-insensitively (#440)

The connect handler returns http.StatusUnsupportedMediaType when a
request has a `Content-Type: application/json; charset=UTF-8` header.
However, according to [RFC 9110 Section 8.3.2](
https://httpwg.org/specs/rfc9110.html#rfc.section.8.3.2), the charset
parameter value should be treated as case-insensitive.

In this PR, I have modified the charset parameter value for user
requests and acceptable content types of handlers to all be handled in
lowercase.

handler_ext_test.go

@@ -95,7 +95,7 @@ func TestHandler_ServeHTTP(t *testing.T) {
 			strings.NewReader("{}"),
 		)
 		assert.Nil(t, err)
-		req.Header.Set("Content-Type", "application/json;Charset=utf-8")
+		req.Header.Set("Content-Type", "application/json;Charset=Utf-8")
 		resp, err := client.Do(req)
 		assert.Nil(t, err)
 		defer resp.Body.Close()

protocol.go

@@ -320,5 +320,15 @@ func canonicalizeContentType(ct string) string {
 	if err != nil {
 		return ct
 	}
+
+	// According to RFC 9110 Section 8.3.2, the charset parameter value should be treated as case-insensitive.
+	// mime.FormatMediaType canonicalizes parameter names, but not parameter values,
+	// because the case sensitivity of a parameter value depends on its semantics.
+	// Therefore, the charset parameter value should be canonicalized here.
+	// ref.) https://httpwg.org/specs/rfc9110.html#rfc.section.8.3.2
+	if charset, ok := params["charset"]; ok {
+		params["charset"] = strings.ToLower(charset)
+	}
+
 	return mime.FormatMediaType(base, params)
 }

protocol_connect.go

@@ -56,10 +56,10 @@ func (*protocolConnect) NewHandler(params *protocolHandlerParams) protocolHandle
 	contentTypes := make(map[string]struct{})
 	for _, name := range params.Codecs.Names() {
 		if params.Spec.StreamType == StreamTypeUnary {
-			contentTypes[connectUnaryContentTypePrefix+name] = struct{}{}
+			contentTypes[canonicalizeContentType(connectUnaryContentTypePrefix+name)] = struct{}{}
 			continue
 		}
-		contentTypes[connectStreamingContentTypePrefix+name] = struct{}{}
+		contentTypes[canonicalizeContentType(connectStreamingContentTypePrefix+name)] = struct{}{}
 	}
 	return &connectHandler{
 		protocolHandlerParams: *params,

protocol_grpc.go

@@ -85,7 +85,7 @@ func (g *protocolGRPC) NewHandler(params *protocolHandlerParams) protocolHandler
 	}
 	contentTypes := make(map[string]struct{})
 	for _, name := range params.Codecs.Names() {
-		contentTypes[prefix+name] = struct{}{}
+		contentTypes[canonicalizeContentType(prefix+name)] = struct{}{}
 	}
 	if params.Codecs.Get(codecNameProto) != nil {
 		contentTypes[bare] = struct{}{}

protocol_test.go

@@ -0,0 +1,40 @@
+// Copyright 2021-2023 Buf Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package connect
+
+import (
+	"testing"
+
+	"github.com/bufbuild/connect-go/internal/assert"
+)
+
+func TestCanonicalizeContentType(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		arg  string
+		want string
+	}{
+		{name: "charset param should be treated as lowercase", arg: "application/json; charset=UTF-8", want: "application/json; charset=utf-8"},
+		{name: "non charset param should not be changed", arg: "multipart/form-data; boundary=fooBar", want: "multipart/form-data; boundary=fooBar"},
+	}
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, canonicalizeContentType(tt.arg), tt.want)
+		})
+	}
+}