Setting a naming strategy causes private fields to be marshaled

I would expect private fields to only be included if I have called extra.SupportPrivateFields()

```go
	type s struct {
		FieldName string
		privateField string
	}
	s1 := s{FieldName: "foo", privateField:"bar"}
	extra.SetNamingStrategy(extra.LowerCaseWithUnderscores)
	json1, _ := jsoniter.Marshal(s1)
	println(string(json1))

```
output:
`{"field_name":"foo","private_field":"bar"}`