Fix JSONFormat to ensure outputting JSON on NaN and Infinities

[itchyny]

Currently (v1.6.1) JSONFormat emits non-JSON on NaN and Infinities.
This PR fixes JSONFormat to ensure the output to be JSON on NaN and Infinities.
Also added tests for Logfmt and PlainFormat to cover these values (behavior not changed).

Sample code

package main

import (
	"encoding/json"
	"fmt"
	"math"
	"os"
	"time"

	"github.com/cybozu-go/log"
)

func main() {
	var err error
	l := log.NewLogger()
	f := log.JSONFormat{Utsname: "test"}
	t := time.Now()

	buf := make([]byte, 0, 256)
	buf, err = f.Format(buf, l, t, log.LvDebug, "test", map[string]interface{}{
		"x": math.NaN(),
		"y": math.Inf(1),
		"z": math.Inf(-1),
	})
	if err != nil {
		fmt.Fprintf(os.Stderr, "%s\n", err)
		os.Exit(1)
	}
	fmt.Printf("%s\n", buf)

	var data interface{}
	err = json.Unmarshal(buf, &data)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%s\n", err)
		os.Exit(1)
	}
	fmt.Printf("%v\n", data)
}

Result

{"topic":"main","logged_at":"2022-03-14T01:37:50.253201Z","severity":"debug","utsname":"test","message":"test","x":NaN,"y":+Inf,"z":-Inf}

invalid character 'N' looking for beginning of value
exit status 1

Expected behavior

I expect this logger either to emit valid JSON or to return error.
Considering the expectation against loggers, I think it is better to emit JSON to allow restoration of the original data.

• Return an error just like encoding/json: Losing log data on invalid data is unacceptable as a logger.
• Output "NaN", "+Inf", "-Inf" like zap: Allows restoration of the original data, except for the confusion against string values.
  • This PR implements this behavior.
• Output null, 1.7976931348623157e+308, -1.7976931348623157e+308 like jq: Familiar behavior of jq user, and keeps the number type, but cannot distinguish null and NaN and this is confusing.

[morimoto-cybozu]

Sorry for toooooooo late comments.
If you've lost interest in this PR, please let me know. I'll add a small fix and finish this PR.

[morimoto-cybozu]

LGTM
Thanks!