fix codec.Frame in case of flatten struct tag

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

json/options.go

@@ -0,0 +1,17 @@
+package json
+
+import (
+	codec "github.com/unistack-org/micro/v3/codec"
+)
+
+type unmarshalOptionsKey struct{}
+
+func UnmarshalOptions(o JsonUnmarshalOptions) codec.Option {
+	return codec.SetOption(unmarshalOptionsKey{}, o)
+}
+
+type marshalOptionsKey struct{}
+
+func MarshalOptions(o JsonMarshalOptions) codec.Option {
+	return codec.SetOption(marshalOptionsKey{}, o)
+}