allow to flatten struct

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

codec_test.go

@@ -5,6 +5,33 @@ import (
 	"testing"
 )
 
+func TestStructByTag(t *testing.T) {
+	type Str struct {
+		Name []string `json:"name" codec:"flatten"`
+	}
+
+	val := &Str{Name: []string{"first", "second"}}
+
+	c := NewCodec()
+	buf, err := c.Marshal(val)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Equal(buf, []byte(`["first","second"]`)) {
+		t.Fatalf("invalid marshal: %s", buf)
+	}
+
+	err = c.Unmarshal([]byte(`["1","2"]`), val)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(val.Name) != 2 {
+		t.Fatalf("invalid unmarshal: %v", val)
+	}
+
+}
+
 func TestReadBody(t *testing.T) {
 	s := &struct {
 		Name string