Fix updateHeaderExtension for cloned MediaEngine

It addresses the issue that unable to updateHeaderExtension for cloned
MediaEngine, which is resulted from previous commit:

c8b7aa3

mediaengine.go

@@ -284,11 +284,15 @@ func (m *MediaEngine) getHeaderExtensionID(extension RTPHeaderExtensionCapabilit
 // copy copies any user modifiable state of the MediaEngine
 // all internal state is reset
 func (m *MediaEngine) copy() *MediaEngine {
-	return &MediaEngine{
+	cloned := &MediaEngine{
 		videoCodecs:      append([]RTPCodecParameters{}, m.videoCodecs...),
 		audioCodecs:      append([]RTPCodecParameters{}, m.audioCodecs...),
 		headerExtensions: append([]mediaEngineHeaderExtension{}, m.headerExtensions...),
 	}
+	if len(m.headerExtensions) > 0 {
+		cloned.negotiatedHeaderExtensions = map[int]mediaEngineHeaderExtension{}
+	}
+	return cloned
 }
 
 func (m *MediaEngine) getCodecByPayload(payloadType PayloadType) (RTPCodecParameters, RTPCodecType, error) {

mediaengine_test.go

@@ -261,3 +261,28 @@ func TestMediaEngineDoubleRegister(t *testing.T) {
 
 	assert.Equal(t, len(m.audioCodecs), 1)
 }
+
+// The cloned MediaEngine instance should be able to update negotiated header extensions.
+func TestUpdateHeaderExtenstionToClonedMediaEngine(t *testing.T) {
+	src := MediaEngine{}
+
+	assert.NoError(t, src.RegisterCodec(
+		RTPCodecParameters{
+			RTPCodecCapability: RTPCodecCapability{MimeTypeOpus, 48000, 0, "", nil},
+			PayloadType:        111,
+		}, RTPCodecTypeAudio))
+
+	assert.NoError(t, src.RegisterHeaderExtension(RTPHeaderExtensionCapability{"test-extension"}, RTPCodecTypeAudio))
+
+	validate := func(m *MediaEngine) {
+		assert.NoError(t, m.updateHeaderExtension(2, "test-extension", RTPCodecTypeAudio))
+
+		id, audioNegotiated, videoNegotiated := m.getHeaderExtensionID(RTPHeaderExtensionCapability{URI: "test-extension"})
+		assert.Equal(t, 2, id)
+		assert.True(t, audioNegotiated)
+		assert.False(t, videoNegotiated)
+	}
+
+	validate(&src)
+	validate(src.copy())
+}