fix codec.Frame in case of flatten struct tag

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

json.go

@@ -43,10 +43,6 @@ func (c *jsonCodec) Marshal(v interface{}, opts ...codec.Option) ([]byte, error)
 		return nil, nil
 	}
 
-	if m, ok := v.(*codec.Frame); ok {
-		return m.Data, nil
-	}
-
 	options := c.opts
 	for _, o := range opts {
 		o(&options)
@@ -56,6 +52,10 @@ func (c *jsonCodec) Marshal(v interface{}, opts ...codec.Option) ([]byte, error)
 		v = nv
 	}
 
+	if m, ok := v.(*codec.Frame); ok {
+		return m.Data, nil
+	}
+
 	marshalOptions := DefaultMarshalOptions
 	if options.Context != nil {
 		if f, ok := options.Context.Value(marshalOptionsKey{}).(JsonMarshalOptions); ok {
@@ -80,11 +80,6 @@ func (c *jsonCodec) Unmarshal(b []byte, v interface{}, opts ...codec.Option) err
 		return nil
 	}
 
-	if m, ok := v.(*codec.Frame); ok {
-		m.Data = b
-		return nil
-	}
-
 	options := c.opts
 	for _, o := range opts {
 		o(&options)
@@ -94,6 +89,11 @@ func (c *jsonCodec) Unmarshal(b []byte, v interface{}, opts ...codec.Option) err
 		v = nv
 	}
 
+	if m, ok := v.(*codec.Frame); ok {
+		m.Data = b
+		return nil
+	}
+
 	unmarshalOptions := DefaultUnmarshalOptions
 	if options.Context != nil {
 		if f, ok := options.Context.Value(unmarshalOptionsKey{}).(JsonUnmarshalOptions); ok {