Marshalling fails with Interface members when they are nil

[joeblubaugh]

I was migrating some marshalling code to use this library over the standard library (thank you for making it!) and encountered this error. Note that the stdlib reports a nil error with data, while segmentio/encoding/json returns an error. i

Minimal repro:

package main

import (
	"fmt"

	"github.com/segmentio/encoding/json"
)

type segmentJSON struct {
	Stringer fmt.Stringer
	Field    int `json:"MyInt"`
}

func main() {
	fmt.Println("segment-error")
	js := segmentJSON{Field: 2}
	fmt.Println(json.Marshal(js))
}

Output when using encoding/json:

❯ go run main.go
segment-error
[123 34 83 116 114 105 110 103 101 114 34 58 110 117 108 108 44 34 77 121 73 110 116 34 58 50 125] <nil>

with github.com/segmentio/encoding/json:

❯ go run main.go
segment-error
[] json: unsupported type: fmt.Stringer

[achille-roussel]

@joeblubaugh thanks for reporting it the issue, looking into it.