fix #291 omit empty was not handled properly for json raw message

value_tests/raw_message_test.go

@@ -1,10 +1,18 @@
 package test
 
-import "encoding/json"
+import (
+	"encoding/json"
+)
 
 func init() {
 	marshalCases = append(marshalCases,
 		json.RawMessage("{}"),
+		selectedMarshalCase{struct {
+			Env   string          `json:"env"`
+			Extra json.RawMessage `json:"extra,omitempty"`
+		}{
+			Env: "jfdk",
+		}},
 	)
 	unmarshalCases = append(unmarshalCases, unmarshalCase{
 		ptr:   (*json.RawMessage)(nil),