Why (*simplejson.Json).UnmarshalJSON abandon old content in it?

[tipybara]

In Standard Library "encoding/json", do json.Unmarshal(bytes, target) multiple times with different bytes, only duplicate keys will be overwritten by newer one, other keys are always kept.

 var j, _ = simplejson.NewJson([]byte(`{"a":"a"}`))
 j.UnmarshalJSON([]byte(`{"b":"b"}`))
 var b, _ = j.MarshalJSON()
 fmt.Println(string(b)) // {"b": "b"}

 var v = new(V)
 json.Unmarshal([]byte(`{"a":"a"}`), v)
 json.Unmarshal([]byte(`{"b":"b"}`), v)
 b, _ = json.Marshal(v)
 fmt.Println(string(b)) // {"a": "a", "b": "b"}

How about providing a new method like that?