`namespace-uri()` returns the wrong value for attributes

[Arthur Darcet]

For attributes, namespace-uri() always returns the NS of the parent tag:

var data = `<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="root-ns" xmlns:nested="nested-ns">
	<tag nested:attr="val"></tag>
</root>
`

func main() {
	buf := strings.NewReader(data)
	doc, _ := xmlquery.Parse(buf)
	fmt.Println(len(xmlquery.Find(doc, "//*[namespace-uri()='root-ns' and local-name()='tag']")))        // 1
	fmt.Println(len(xmlquery.Find(doc, "//*[@*[namespace-uri()='nested-ns' and local-name()='attr']]"))) // 0
	fmt.Println(len(xmlquery.Find(doc, "//*[@*[namespace-uri()='root-ns' and local-name()='attr']]")))   // 1
}

should output 1 1 0, but returns 1 0 1