protocol/rtp: added TestExtHeaderLen

Added test TestExtHeaderLen which checks that extHeaderLen returns the correct len for
an RTP packet with an extension header.

protocol/rtp/parse_test.go

@@ -82,3 +82,20 @@ func TestHasExt(t *testing.T) {
 		t.Error("did not expect to have extension indicator as true")
 	}
 }
+
+// TestExtHeaderLen checks for a correct extension header len for an RTP packet.
+func TestExtHeaderLen(t *testing.T) {
+	const ver, expect = 2, 3
+	pkt := (&Pkt{
+		V: ver,
+		X: true,
+		Extension: ExtensionHeader{
+			ID:     0,
+			Header: make([][4]byte, expect),
+		},
+	}).Bytes(nil)
+	got := extHeaderLen(pkt)
+	if got != expect {
+		t.Errorf("Unexpected extension header len. Got: %v\n Want: %v\n", got, expect)
+	}
+}