Fixed a bug where ampersand character (&) was escaped in Child operators

[rjgpacheco]

This PR fixes what seems to be a bug in the string serialisation for expressions.

The ampersand & character got escaped into \h which, besides not being the right string, also isn't a valid according to jp.ParseString, eg (https://go.dev/play/p/BDjHacoE4Hu):

package main

import (
	"fmt"
	"github.com/ohler55/ojg/jp"
)

func main() {
	original := jp.R().C("a & b")
	_, err := jp.ParseString(original.BracketString())
	fmt.Println(err)
}

Shows the error:

0x68 (h) is not a valid escaped character

I've added some basic tests covering this, but I'm unfamiliar with other parts of the code. I think the only place this is relevant is in

ojg/string.go

Lines 61 to 69 in aede9ec

	case 'h':
	if htmlSafe {
	if start < i {
	buf = append(buf, s[start:i]...)
	}
	buf = append(buf, `\u00`...)
	buf = append(buf, hex[(b>>4)&0x0f])
	buf = append(buf, hex[b&0x0f])
	start = i + 1

, where it seems to be parting HTML, where & is special. I think the mapping got copied over and this bug sneaked in.

[ohler55]

Great catch and fix. Thank you.