change enum MarshalJSON() method to not be called on ptr

[tiehm]

We have a bug in the current enum rendering system where the MarshalJSON method is never called on pointer structs. This fixes this behavior (quiet easily).

import (
    "encoding/json"
    "fmt"
)

type Enum struct {
    value string
}

type Enum2 struct {
    valie string
}

func (e Enum) MarshalJSON() ([]byte, error) {
    return []byte("\"another_value\""), nil
}

func (e *Enum2) MarshalJSON() ([]byte, error) {
    return []byte("\"another_value\""), nil
}

type Test struct {
    A Enum
    C Enum2
    B int
}

func main() {
    x := Test{
        A: Enum{},
        C: Enum2{},
        B: 1,
    }
    j, err := json.Marshal(x)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(j))
}

Prints:

{"A":"another_value","C":{},"B":1}

Go playground example: https://go.dev/play/p/8QQzFwO9a-K