Fix ,string option applying to non applicable value / types

[ngicks]

This is a PR that explores how to fix #657 (comment).

The json:",string" option incorrectly quotes value even when it must not be applied.

If this patch is merged, it would not quote nil, or types other than string, boolean, numeric and pointer types that points to those. Also it would report error correctly.

package main

import (
	"encoding/json"
	"fmt"

	jsoniter "github.com/json-iterator/go"
)

type quote struct {
	S *string `json:",string"`
}

func main() {
	var (
		q   quote
		err error
	)
	for idx, str := range []string{
		`{}`,
		`{"S":"\"234\""}`,
		`{"S":"\"\""}`,
		`{"S":"\""}`,
		`{"S":"234"}`,
		`{"S":234"}`,
		`{"S":"234}`,
	} {
		fmt.Printf("case %d, %s\n", idx, str)
		err = jsoniter.ConfigCompatibleWithStandardLibrary.Unmarshal([]byte(str), &q)
		fmt.Printf("jsoniter err: %+v\n", err)
		err = json.Unmarshal([]byte(str), &q)
		fmt.Printf("json err: %+v\n", err)
		fmt.Println()
	}
}

/*
case 0, {}
jsoniter err: <nil>
json err: <nil>

case 1, {"S":"\"234\""}
jsoniter err: <nil>
json err: <nil>

case 2, {"S":"\"\""}
jsoniter err: <nil>
json err: <nil>

case 3, {"S":"\""}
jsoniter err: main.quote.S: stringModeStringDecoder: expect len(s) >= 2, but found ", error found in #9 byte of ...|{"S":"\""}|..., bigger context ...|{"S":"\""}|...
json err: json: invalid use of ,string struct tag, trying to unmarshal "\"" into string

case 4, {"S":"234"}
jsoniter err: main.quote.S: stringModeStringDecoder: expect ", but found 2, error found in #10 byte of ...|{"S":"234"}|..., bigger context ...|{"S":"234"}|...
json err: json: invalid use of ,string struct tag, trying to unmarshal "234" into string

case 5, {"S":234"}
jsoniter err: main.quote.S: ReadString: expects " or n, but found 2, error found in #6 byte of ...|{"S":234"}|..., bigger context ...|{"S":234"}|...
json err: invalid character '"' after object key:value pair

case 6, {"S":"234}
jsoniter err: main.quote.S: readStringSlowPath: unexpected end of input, error found in #10 byte of ...|{"S":"234}|..., bigger context ...|{"S":"234}|...
json err: unexpected end of JSON input
*/