fix: Use value receiver for MarshalJSON

[alexandear]

Consistently use value receivers for MarshalJSON methods.

The issue is clearly illustrated by the following program, where panic is never called. However, if we change p := Person{"Bob"} to p := &Person{"Bob"}, everything works as expected, and the panic occurs.

package main

import (
	"encoding/json"
	"fmt"
)

type Person struct {
	Name string
}

func (p *Person) MarshalJSON() ([]byte, error) {
	panic("unexpected")
}

func main() {
	p := Person{"Bob"}
	b, err := json.Marshal(p)
	if err != nil {
		panic(err)
	}
	fmt.Printf("%s\n", b)
}

The issue exists only in encoding/json v1. The experimental encoding/json/v2 package does not have this issue.

See https://go.dev/issue/22967 for more details.

I think this is a non-breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.69%. Comparing base (a5eca73) to head (7c033db).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4211   +/-   ##
=======================================
  Coverage   93.69%   93.69%           
=======================================
  Files         209      209           
  Lines       19772    19772           
=======================================
  Hits        18526    18526           
  Misses       1049     1049           
  Partials      197      197           

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[munlicode]

LGTM

[gmlewis]

Thank you, @alexandear and @munlicode!
LGTM.
Merging.